Apply indexing configuration to ruby-lsp-doctor

[andyw8]

Motivation

Due to the changes in #1814, the ruby-lsp-doctor tool needs to have the configuration loaded.

Implementation

Check if an .index.yml exists, and load configuration if so.

Automated Tests

Since this is a troubleshooting tool, I think tests are not critical.

Manual Tests

• Create a file "foo/bar.rb"
• Run the index and pipe through grep to confirm foo/bar.rb is listed
• Update .index.yml to exclude "**/foo**"
• Run the index and pipe through grep to confirm foo/bar.rb is no longer listed

Notes:

• This will need adjusted again for Introduce .ruby-lsp.yml config file #1434
• I think I found a separate problem relating to relative paths while testing this. I will look into and create an issue if needed.

[vinistock]

This may file to parse. Let's rescue here and warn if the YAML file is invalid.

[andyw8]

I've made it abort instead of warn, since a warn might be easy to miss among the other output.