[Core] Add Chordal Hold, an "opposite hands rule" tap-hold option similar to Achordion, Bilateral Combinations. (#24560)

* Chordal Hold: restrict what chords settle as hold

* Chordal Hold: docs and further improvements

* Fix formatting.

* Doc rewording and minor edit.

* Support Chordal Hold of multiple tap-hold keys.

* Fix formatting.

* Simplification and additional test.

* Fix formatting.

* Tighten tests.

* Add test two_mod_taps_same_hand_hold_til_timeout.

* Revise handing of pairs of tap-hold keys.

* Generate a default chordal_hold_layout.

* Document chordal_hold_handedness().

* Add license notice to new and branched files in PR.

* Add `tapping.chordal_hold` property for info.json.

* Update docs/reference_info_json.md

* Revise "hand" jsonschema.

* Chordal Hold: Improved layout handedness heuristic.

This commit improves the heuristic used in generate-keyboard-c for
inferring key handedness from keyboard.json geometry data.

Heuristic summary:

1. If the layout is symmetric (e.g. most split keyboards), guess the
   handedness based on the sign of (x - layout_x_midpoint).

2. Otherwise, if the layout has a key of >=6u width, it is probably the
   spacebar. Form a dividing line through the spacebar, nearly vertical
   but with a slight angle to follow typical row stagger.

3. Otherwise, assume handedness based on the widest horizontal
   separation.

I have tested this strategy on a couple dozen keyboards and found it to
work reliably.

* Use Optional instead of `| None`.

* Refactor to avoid lambdas.

* Remove trailing comma in chordal_hold_layout.

* Minor docs edits.

* Revise to allow combining multiple same-hand mods.

This commit revises Chordal Hold as described in discussion in
https://github.com/qmk/qmk_firmware/pull/24560#discussion_r1894655238

1. In "RCTL_T(KC_A)↓, RSFT_T(KC_C)↓, RCTL_T(KC_A)↑" before the tapping
   term, RCTL_T(KC_A) is settled as tapped.
2. In "RCTL_T(KC_A)↓, RSFT_T(KC_C)↓, RSFT_T(KC_C)↑", both RCTL_T(KC_A)
   and RSFT_T(KC_C) are settled as tapped.
3. In "RCTL_T(KC_A)↓, RSFT_T(KC_C)↓, KC_U↓" (all keys on the same side),
   both RCTL_T(KC_A) and RSFT_T(KC_C) are settled as tapped.
4. In "RCTL_T(KC_A)↓, RSFT_T(KC_C)↓, LSFT_T(KC_T)↓", with the third key
   on the other side, we allow Permissive Hold or Hold On Other Keypress
   to decide how/when to settle the keys.
5. In "RCTL_T(KC_A)↓, RSFT_T(KC_C)↓" held until the tapping term, the
   keys are settled as held.

1–3 provide same-hand roll protection. 4–5 are for combining multiple
same-hand modifiers.

I've updated the unit tests and have been running it on my keyboard, for
a few hours so far, and all seems good. I really like this scheme. It
allows combining same-side mods, yet it also has roll protection on
streaks. For me, this feels like Achordion, but clearly better streak
handling and improved responsiveness.

* Fix formatting.

* Add a couple tests with LT keys.

* Remove stale use of CHORDAL_HOLD_LAYOUT.

* Fix misspelling lastest -> latest

* Handling tweak for LTs and tests.

* Fix formatting.

* More tests with LT keys.

* Fix formatting.

data/mappings/info_config.hjson

@@ -200,6 +200,7 @@
     "SPLIT_WPM_ENABLE": {"info_key": "split.transport.sync.wpm", "value_type": "flag"},
 
     // Tapping
+    "CHORDAL_HOLD": {"info_key": "tapping.chordal_hold", "value_type": "flag"},
     "HOLD_ON_OTHER_KEY_PRESS": {"info_key": "tapping.hold_on_other_key_press", "value_type": "flag"},
     "HOLD_ON_OTHER_KEY_PRESS_PER_KEY": {"info_key": "tapping.hold_on_other_key_press_per_key", "value_type": "flag"},
     "PERMISSIVE_HOLD": {"info_key": "tapping.permissive_hold", "value_type": "flag"},

data/schemas/keyboard.jsonschema

@@ -422,7 +422,11 @@
                                 "h": {"$ref": "qmk.definitions.v1#/key_unit"},
                                 "w": {"$ref": "qmk.definitions.v1#/key_unit"},
                                 "x": {"$ref": "qmk.definitions.v1#/key_unit"},
-                                "y": {"$ref": "qmk.definitions.v1#/key_unit"}
+                                "y": {"$ref": "qmk.definitions.v1#/key_unit"},
+                                "hand": {
+                                    "type": "string",
+                                    "enum": ["L", "R", "*"]
+                                }
                             }
                         }
                     }
@@ -915,6 +919,7 @@
         "tapping": {
             "type": "object",
             "properties": {
+                "chordal_hold": {"type": "boolean"},
                 "force_hold": {"type": "boolean"},
                 "force_hold_per_key": {"type": "boolean"},
                 "ignore_mod_tap_interrupt": {"type": "boolean"},