Issue Specific Errors for Common Cases

[InsertCreativityHere]

With the new parser, we emit a general syntax that describes what the parser found, and what valid things should of been there instead. This is fine for general cases, but there are many common syntax errors that we should add more specific error messages for. For instance:

class MyClass {} // Must be inside a module.

struct Foo: Bar {} // Structs don't support inheritance

struct Foo {}
[[attribute]] // Probably should be a local attribute
struct Bar {}

Striking the right balance between addressing common cases without over-doing it will be non-trivial.
But if anyone runs into a common syntax error that they think warrants a specific error message, feel free to post it here!

[ReeceHumphreys]

Another good error to emit would be if you are using a keyword somewhere by mistake for example:

...
interface I {
    custom();
}

This gives the following error message:

error: expected one of "[", "}", doc_comment, idempotent_keyword, identifier, but found 'CustomKeyword'
 --> foo.slice:4:5
  |
4 |     custom();
  |     ------
  |

Failed: Compilation failed with 1 error(s)

where it would generally be more helpful to have an error message along the lines of you used the keyword custom instead of a valid identifier. Perhaps you want to escape it with \custom ...

[ReeceHumphreys]

Found another example with the strings_invalid_as_tag_value test. The error emitted for the below slice is expected one of \"-\", integer_literal, but found 'StringLiteral(\"test string\")'. Not very clear.

testOp(a: tag(\"test string\") int32?);

[ReeceHumphreys]

Once #510 is merged we now have introduced an error for when doc comments are used on parameters, modules, and return memebers. This error would also be a good candidate for ErrorRecovery with a specific message.

[InsertCreativityHere]

For attributes we should add notes about what an attribute can be applied to when it's put on the wrong thing.