zed: add option to override settings

[sweenu]

Description

It's great to be able to merge the config and for the settings files not to be read only, so that we can use all of Zed's features.
However, it can make it a pain when wanting to configure 100% declaratively.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.
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  • Basic tests added. See Tests

• If this PR adds an exciting new feature or contains a breaking change.

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