programs.wezterm: add settings option

Add a `settings` option that allows WezTerm to be configured
declaratively using a Nix attribute set. Settings are serialized to
Lua using `lib.generators.toLua`. Raw Lua expressions such as
`wezterm.font` and `wezterm.action.*` can be embedded using
`lib.generators.mkLuaInline`.

When `settings` is set, `extraConfig` is still supported. It is
wrapped in an immediately invoked function expression so that any
`return` statement (including the common `return {}` and
`return config` patterns) is captured and merged on top of the
generated settings, preserving full backward compatibility.

The merge between `settings` and `extraConfig` is shallow: a key
returned from `extraConfig` replaces the entire corresponding value
from `settings`, including nested tables and arrays.

Closes #6047

docs/release-notes/rl-2605.md

@@ -7,6 +7,14 @@ section is therefore not final.
 
 This release has the following notable changes:
 
+- WezTerm now supports declarative configuration via
+  [](#opt-programs.wezterm.settings). Settings are expressed as a Nix
+  attribute set and serialized to Lua using `lib.generators.toLua`.
+  Raw Lua expressions such as `wezterm.font` and `wezterm.action.*`
+  can be embedded using `lib.generators.mkLuaInline`. The existing
+  [](#opt-programs.wezterm.extraConfig) option remains fully supported
+  and can be combined with [](#opt-programs.wezterm.settings).
+
 - The [](#opt-programs.anki.uiScale) option now expects a value in the
   range 1.0–2.0, previously it erroneously expected values in the
   range `0.0–1.0`.