fix(compiler): reserved words are not blocked (#3179)

Fixes #3110

This is list is pretty naive, I think we should eventually do some mangling/cleanup of the JS we emit to avoid so many reserved values. Since I believe there's more work to do, I didn't touch the spec yet (See #1964).

This change will at least prevent strange runtime errors due to using reserved words. Added a special case for "constructor" as a method name, since it's not technically reserved elsewhere.

Misc:
- Changed the add_error in the parser to have a version that doesn't return anything. Sorry that this caused a bit of a noisy diff but I was so tired of that dumb `let _ = ...` thing

## Checklist

- [x] Title matches [Winglang's style guide](https://docs.winglang.io/contributing/pull_requests#how-are-pull-request-titles-formatted)
- [x] Description explains motivation and solution
- [x] Tests added (always)
- [ ] Docs updated (only required for features)
- [ ] Added `pr/e2e-full` label if this feature requires end-to-end testing

*By submitting this pull request, I confirm that my contribution is made under the terms of the [Monada Contribution License](https://docs.winglang.io/terms-and-policies/contribution-license.html)*.

examples/tests/invalid/class.w

@@ -157,4 +157,7 @@ inflight class Jet extends Plane{
     super();
 //  ^^^^^^^^ Expected 1 positional argument(s) but got 0
   }
+  constructor() {
+//^^^^^^^^^^^ To declare a initializer, use "init"
+  }
 }

examples/tests/invalid/unknown_symbol.w

@@ -31,3 +31,6 @@ class A extends B {
 
 unknown = 1;
 //^ Unknown symbol
+
+let let = 2;
+  //^^^ Reserved word