[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/c2json.py

@@ -1,7 +1,6 @@
 """Generate a keymap.json from a keymap.c file.
 """
 import json
-import sys
 
 from milc import cli
 
@@ -21,19 +20,14 @@ def c2json(cli):
 
     This command uses the `qmk.keymap` module to generate a keymap.json from a keymap.c file. The generated keymap is written to stdout, or to a file if -o is provided.
     """
-    cli.args.filename = qmk.path.normpath(cli.args.filename)
+    if cli.args.filename != '-':
+        cli.args.filename = qmk.path.normpath(cli.args.filename)
 
-    # Error checking
-    if not cli.args.filename.exists():
-        cli.log.error('C file does not exist!')
-        cli.print_usage()
-        exit(1)
-
-    if str(cli.args.filename) == '-':
-        # TODO(skullydazed/anyone): Read file contents from STDIN
-        cli.log.error('Reading from STDIN is not (yet) supported.')
-        cli.print_usage()
-        exit(1)
+        # Error checking
+        if not cli.args.filename.exists():
+            cli.log.error('C file does not exist!')
+            cli.print_usage()
+            return False
 
     # Environment processing
     if cli.args.output == ('-'):
@@ -47,7 +41,7 @@ def c2json(cli):
         keymap_json = qmk.keymap.generate_json(keymap_json['keymap'], keymap_json['keyboard'], keymap_json['layout'], keymap_json['layers'])
     except KeyError:
         cli.log.error('Something went wrong. Try to use --no-cpp.')
-        sys.exit(1)
+        return False
 
     if cli.args.output:
         cli.args.output.parent.mkdir(parents=True, exist_ok=True)