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
