Merge global config with project config

[Davidyz]

It makes more sense for some config options (like database settings, embedding functions, etc.) to be globally effective. This PR makes it possible. VectorCode CLI/LSP/MCP will now:

1. check for global config at ~/.config/vectorcode/config.json (or json5). If exists, load it.
2. check for project-local config at project_root/.vectorcode/config.json (or json5). If exists, load it and merge it with the global config (local config takes higher precedence).

You can now configure your database, embedding functions, etc., only once in the global config, and they'll be effective even if you have a local config, as long as they don't declare different settings for db/embeddings, etc.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.89%. Comparing base (171361e) to head (db7e74c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
+ Coverage   99.72%   99.89%   +0.16%     
==========================================
  Files          25       25              
  Lines        1845     1845              
==========================================
+ Hits         1840     1843       +3     
+ Misses          5        2       -3     

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