Assign error codes for various GDScript compile errors

[timothyqiu]

In addition to text error messages, add error codes to them. For example LNK2001: unresolved external symbol.

This will make life easier if we finally localize error messages.

[Calinou]

This is a good idea to improve searchability of error messages in general. One open question that remains is, how to name those error codes:

1. Use human-readable English names like unresolved_external_symbol. This immediately gives you an idea of what the error code is about. This appears to be what most linters like ESLint do. GDScript warnings already have those snake_case identifiers for the @warning_ignore() annotation.
2. Alternatively, use error codes like GDS1001. This wouldn't match what GDScript warnings currently do, though.

[Mickeon]

GDScript aims to be very user-friendly, so arbitrary numerical error codes over a human-readable ones may be "alienating", unless they're internally associated to the error specifically for localization purposes.

[timothyqiu]

This is not about replacing Unexpected indentation. with GDS1001, but replacing it with GDS1001: Unexpected indentation..

So we can give localized error messages like GDS1001: Indentation inattendue. and GDS1001: 意外的缩进。, and still allow these users to search for that error in a wider community.