tint: Refactor Extensions / Enables.

* Extract ast::Enable::ExtensionKind to ast::Extension.
* Move the parsing out of ast::Enable and next to ast/extension.h
* Change the ast::Enable constructor to take the Extension, instead of
  a std::string. It's the WGSL parser's responsibility to parse, not the
  AST nodes.
* Add ProgramBuilder::Enable() helper.
* Keep ast::Module simple - keep track of the declared AST Enable nodes,
  don't do any deduplicating of the enabled extensions.
* Add the de-duplicated ast::Extensions to the sem::Module.
* Remove the kInternalExtensionForTesting enum value - we have kF16
  now, which can be used instead for testing.
* Rename kNoExtension to kNone.

Bug: tint:1472
Change-Id: I9af635e95d36991ea468e6e0bf6798bb50937edc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/90523
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/tint/extensions/parsing/basic.wgsl

@@ -0,0 +1,7 @@
+// Enable a void internal extension
+enable f16;
+
+@stage(fragment)
+fn main() -> @location(0) vec4<f32> {
+    return vec4<f32>(0.1, 0.2, 0.3, 0.4);
+}