Fix font axes/features settings UI (#17164)

Due to #16821 everything about #16104 broke. This PR rights the wrongs
by rewriting all the `Font`-based code to not use `Font` at all.
Instead we split the font spec once into font families, do a lot of
complex logic to split font axes/features into used and unused ones
and construct all the UI elements. So. much. boilerplate. code.

Closes #16943

## Validation Steps Performed
There are more edge cases than I can list here... Some ideas:
* Edit the settings.json with invalid axis/feature keys ✅
* ...out of range values ✅
* Settings UI reloads when the settings.json changes ✅
* Adding axes/features works ✅
* Removing axes/features works ✅
* Resetting axes/features works ✅
* Axes/features apply in the renderer when saving ✅

doc/cascadia/profiles.schema.json

@@ -349,7 +349,7 @@
           "description": "Sets the DWrite font features for the given font. For example, { \"ss01\": 1, \"liga\":0 } will enable ss01 and disable ligatures.",
           "type": "object",
           "patternProperties": {
-            "^(([A-Za-z0-9]){4})$": {
+            "^[\\x20-\\x7E]{4}$": {
               "type": "integer"
             }
           },
@@ -359,7 +359,7 @@
           "description": "Sets the DWrite font axes for the given font. For example, { \"wght\": 200 } will set the font weight to 200.",
           "type": "object",
           "patternProperties": {
-            "^([A-Za-z]{4})$": {
+            "^[\\x20-\\x7E]{4}$": {
               "type": "number"
             }
           },