[CLI] Add stdin support for json2c command (#11289)

* Implement stdin for json2c command

* Refactor

* Handle json decode error

* Add stdin support for c2json cli command

* Refactor to prevent code duplication

* Change exit(1) to return False in c2json command

* Remove unused import

lib/python/qmk/cli/json2c.py

@@ -1,6 +1,7 @@
 """Generate a keymap.c from a configurator export.
 """
 import json
+import sys
 
 from milc import cli
 
@@ -17,26 +18,31 @@ def json2c(cli):
 
     This command uses the `qmk.keymap` module to generate a keymap.c from a configurator export. The generated keymap is written to stdout, or to a file if -o is provided.
     """
-    # Error checking
-    if cli.args.filename and cli.args.filename.name == '-':
-        # TODO(skullydazed/anyone): Read file contents from STDIN
-        cli.log.error('Reading from STDIN is not (yet) supported.')
-        cli.print_usage()
-        return False
 
-    if not cli.args.filename.exists():
-        cli.log.error('JSON file does not exist!')
-        cli.print_usage()
+    try:
+        # Parse the configurator from stdin
+        if cli.args.filename and cli.args.filename.name == '-':
+            user_keymap = json.load(sys.stdin)
+
+        else:
+            # Error checking
+            if not cli.args.filename.exists():
+                cli.log.error('JSON file does not exist!')
+                return False
+
+            # Parse the configurator json file
+            else:
+                user_keymap = json.loads(cli.args.filename.read_text())
+
+    except json.decoder.JSONDecodeError as ex:
+        cli.log.error('The JSON input does not appear to be valid.')
+        cli.log.error(ex)
         return False
 
     # Environment processing
     if cli.args.output and cli.args.output.name == '-':
         cli.args.output = None
 
-    # Parse the configurator json
-    with cli.args.filename.open('r') as fd:
-        user_keymap = json.load(fd)
-
     # Generate the keymap
     keymap_c = qmk.keymap.generate_c(user_keymap['keyboard'], user_keymap['layout'], user_keymap['layers'])