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.expected.hlsl

@@ -0,0 +1,14 @@
+struct tint_symbol {
+  float4 value : SV_Target0;
+};
+
+float4 main_inner() {
+  return float4(0.100000001f, 0.200000003f, 0.300000012f, 0.400000006f);
+}
+
+tint_symbol main() {
+  const float4 inner_result = main_inner();
+  tint_symbol wrapper_result = (tint_symbol)0;
+  wrapper_result.value = inner_result;
+  return wrapper_result;
+}