feat: expose diagnostic.code in from_json & from_regex

[SebasF1349]

Expose diagnostic.code in from_json and from_regex for better customizability

closes #243

[xiaoshihou514]

I think this is not backwards compatible, best to keep the defaults.

[SebasF1349]

Sorry, yesterday I had to go and forgot to mention that I didn't fix the tests yet.

It is not backwards compatible in the sense that now the message won't include the error code by default, as seen in the failing test:

Passed in:
(string) 'Very important error message'
Expected:
(string) 'Very important error message[error code 114514]'

Configuration doesn't (shouldn't) need to be changed and the code can be readded by using vim.diagnostic.config if the user prefers. Maybe a note on how to do it should be added to the docs with a simple example.

[xiaoshihou514]

Good point.