[Keymap] crkbd/keymaps/rs: add rs keymap to corne keyboard and adapt others (#5181)

Update all my keymaps to work with 40 keys. Refactor code using
Userspace.

users/rs/readme.md

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+# RS: Code Friendly 40% – 60% Keymaps
+
+The rs keymap collection is an evolution of my previous keymap optimized for coding with a 60% keyboards like the Iris. I tried to keep the simplicity of my previous keymap with all the keys necessary for coding on a single layer in addition to the base one. It work well with any 40% and 60% keyboard, split or not.
+
+To build it, use:
+
+    make <keyboard>:rs
+
+Example:
+
+    make keebio/iris:rs
+    make crkbd:rs
+    make planck:rs
+    make preonic:rs
+    make ergotravel:rs
+    make handwired/rs60:rs
+
+Because I sometime have to use my internal keyboard I my macbook, a [karabiner configuration](karabiner.json) is also provided to get most of the features of this keyboard, including the code layer / backspace on right command key etc.
+
+This set of keymaps have been tested with those keyboards:
+- [Planck](../../keyboards/planck/)
+- [Preonic](../../keyboards/preonic/)
+- [My preonic clone](../../keyboards/handwired/rs60/)
+
+This keymap in is also available for other keyboards:
+- [Crkdb/rs](../../keyboards/crkbd/keymaps/rs/keymap.c)
+- [Iris/rs](../../keyboards/keebio/iris/keymaps/rs/keymap.c)
+- [Ergotravel/rs](../../keyboards/ergotravel/keymaps/rs/keymap.c)
+- [ortho_5x12/rs](../../layouts/community/ortho_5x12/rs/keymap.c)
+- [ortho_4x12/rs](../../layouts/community/ortho_4x12/rs/keymap.c)