kitty: allow not setting shell_integration

[MithicSpirit]

Description

This allows better support for using kitty's built-in shell integration
method by setting shellIntegration.mode to null, which makes it not
be set at all in the config file and disables modification to shell
initialization scripts by default. If this is used, the user should set
settings.shell_integration instead (if shell integration is desired),
which receives no special handling.

I'm not sure whether the mode != null should just be part of the default options or if it should be forced.

Fixes #5809

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• Test cases updated/added. See example.

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• If this PR adds a new module

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Maintainer CC

@teto

[MithicSpirit]

At this rate, maybe I should just add myself as a maintainer.

[khaneliman]

At this rate, maybe I should just add myself as a maintainer.

Thought always crosses my mind lmao