Fix for 3145: get mkindex.py to list out test case files in subdirectories

[kaby76]

Fix for #3145

_scripts/mkindex.py script changes:

• Changed os.libdir() to Path(), so example files in subdirectories are listed.
• Added checks for ".errors" and ".tree" files, so they are not listed in lab.antlr.org as an input file to parse. Python 3.10 is now required.
• Added code to replace backslashes in paths to forward slashes.
• Added code to format json file, so we can read the results.

Grammars.json file:

• A number of grammars were not listed, start symbols wrong, and otherwise new test file examples added over the last few months to the repo. I updated the grammars.json file to support the current version of the "Antlr lab" running at lab.antlr.org.

To do:

• Add grammars-v4 dir indexer #2866 should be closed. @parrt
• We still need to do lab.antlr.org should reference grammars.json as a Github Actions artifact in gramamrs-v4 antlr4-lab#52. To do that, we'll need a fine-grained personal access token for this repo. It should have only read access.

[kaby76]

Path names should be relative to the examples directory. It's currently relative to the root directory. It should not.

[teverett]

@kaby76 thanks!

[parrt]

#2866 should be closed.

Done.

Regarding the access token... I guess that's something I need to feed to my server when I start it up? Are you sure there's no way I can just download via regular http? This is friction to my world and I'm already barely hanging on in terms of my workload.