[RCTUITextView] Remove some magic constants

[Saadnajmi]

Summary:

RCTUITextView has two instances where it references some magic constants for font size and color. We inherited these in React Native macOS, and now I want to remove that diff & implement these properly :D.

Looking at commit history, these constants were found by UI inspecting a UITextView several years ago. We don't need to do this.. Apple provides API's for accessing these constants through the system. This gives us slightly better support for things like Large Text support (where 17 needs to be scaled) and Dark Mode (where the old placeholderFontColor is just wrong). Granted, these constants are overwritten almost immediately, I still thought it worth using the proper OS APIs.

The other nice thing about this API is they have very nice macOS equivalents, making the diffs nicer :).

Changelog:

Pick one each for the category and type tags:

[IOS] [FIXED] - Remove some magic constants from RCTUITextView

Test Plan:

Verified that the default UIFont is still 17 after this change.

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	8,755,252	+2
android	hermes	armeabi-v7a	8,067,828	+3
android	hermes	x86	9,247,855	+4
android	hermes	x86_64	9,096,987	+3
android	jsc	arm64-v8a	9,316,460	+2
android	jsc	armeabi-v7a	8,506,362	+5
android	jsc	x86	9,379,948	+4
android	jsc	x86_64	9,633,188	+3

Base commit: af4a0336e95ee3f9a43b96acfb09595c46e22fa4 Branch: main

[facebook-github-bot]

@dmytrorykun has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@dmytrorykun merged this pull request in facebook/react-native@a4a8b4d8eb7ce9c50afc6061362d67777f60b2df.