WebGPU: Replace UChar with char16_t

[adetaylor]

7f8fedb2c66fe622eaf9790af7a32bc478692aa2

WebGPU: Replace UChar with char16_t
https://bugs.webkit.org/show_bug.cgi?id=294730
rdar://153822449

Reviewed by NOBODY (OOPS!).

WebKit wishes to move from UChar to the standard C++ chat16_t. This PR makes
the change in Source/WebGPU; it's a simple textual replacement of \bUChar\b
with char16_t. These are typedef'd to be the same so no functional differences
are expected.

* Source/WebGPU/WGSL/Lexer.cpp:
(WGSL::isIdentifierStart):
(WGSL::isIdentifierContinue):
* Source/WebGPU/WGSL/Lexer.h:
(WGSL::Lexer::Lexer):
* Source/WebGPU/WGSL/Parser.cpp:
(WGSL::parse):

https://github.com/WebKit/WebKit/commit/7f8fedb2c66fe622eaf9790af7a32bc478692aa2

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win
	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2	⏳ 🧪 win-tests
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt		✅ 🛠 wpe-cairo
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 vision	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	✅ 🧪 api-gtk
	✅ 🧪 vision-wk2	✅ 🧪 mac-intel-wk2	~~🛠 playstation~~
	✅ 🛠 tv	✅ 🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on current version of this PR (hash https://github.com/WebKit/WebKit/commit/7f8fedb2c66fe622eaf9790af7a32bc478692aa2)

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win
	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2	⏳ 🧪 win-tests
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt		✅ 🛠 wpe-cairo
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 vision	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	✅ 🧪 api-gtk
	✅ 🧪 vision-wk2	✅ 🧪 mac-intel-wk2	~~🛠 playstation~~
✅ 🛠 🧪 unsafe-merge	✅ 🛠 tv	✅ 🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-commit-queue]

Committed 296800@main (f3097d84b527): https://commits.webkit.org/296800@main

Reviewed commits have been landed. Closing PR #46965 and removing active labels.