Add AddMissingEqualsToTypeDefinition code fixer

[cartermp]

Another common error that gets people: #1121

Also works for DU and type abbreviation declarations.

meow

[cartermp]

This is ready for review

[KevinRansom]

looking for '=' in the error text seems a bit too hit or miss.

[cartermp]

Okay, this one is fine as-is but a better approach is to make the error code unique rather than filter based on a heuristic

[cartermp]

Okay @KevinRansom I changed this to only trigger on the specific parse error that we raise for type declarations.

I'll likely look into a separate error message for class declarations in a different PR, and then add that to this fixer only when that specific error message is available.

This will avoid the heuristic of checking the contents of the message and avoid the case that @baronfel found.

[smoothdeveloper]

@cartermp should those code fixes also come with few tests to at least highlight the happy case?

This will avoid regressions of the happy case, and if later on we find false positive or bugs, having tests for positive case provides aproximate exemple on how to write such test.

I feel this would apply to all the code fixes PR that came recently.