[config] add the possibility to include specific files / directories for scanning

[joelwurtz]

I use this lsp for some times now, and i also use a lot this tool https://github.com/jolicode/castor/ which creates a .castor.stub.php at the root of your projects and we can also have a .castor with php files inside.

Those files / directories where currently ignored due to hidden / gitgnore skips in scanning. I added a config under [indexing] that allow to include any file / directory we want.

They are indexed after the workspace scan, so they fill gaps rather than displacing what it found. The default is unchanged.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, examples/)
• I have updated the config schema (config-schema.json)
• I have added/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.

[joelwurtz]

For the record, i rebase to try the "full" setting from 0.10 release but it seems it still does not look at gitignored files / hidden directory by default (so still need this) but maybe i'm wrong ? (did not look at the implementation)