feat: add "languages" configuration option

[dkhokhlov]

Allows overriding grep_ast's file extension to language mapping through aider config. This enables proper language detection for files that may conflict with default mappings.
Key changes:

• add languages section to config schema
• add command line arg --languages to specify mappings
• override grep_ast's PARSERS dict with user-defined mappings
• support both Python dict and JSON formats via ast.literal_eval
• update documentation

Example usage:

# In .aider.conf.yml
languages:
  m: matlab    # treat .m as MATLAB instead of Objective-C
  r: R              # specify R for .r files

Or via command line:

aider --languages "{'m':'matlab', 'r':'R'}"

This enables proper language support for files that may conflict with grep_ast's default mappings, like .m files which default to Objective-C vs Matlab. The mapping can be used with any language supported by py-tree-sitter-languages package, including those added in future versions.

Testing:

• tested with new py-tree-sitter-languages that includes Matlab support using wheels from this GH build.

[CLAassistant]

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