Improve primitive types

[pluralia]

• Add bigint as a built-in type in grammar and make date with a capital letter

PrimitiveType returns string:
    'string' | 'number' | 'boolean' | 'Date' | 'bigint';

• Validate bigint as a return type of terminal rules

[pluralia]

Duplicates #199, that also can be considered as a follow-up where it's necessary to implement the default value converter for bigint and date.

[montymxb]

I can take this one