Fixes#4631
This introduces a mechanism by which parsed config fields can be renamed
to maintain backwards compatibility. This already has a use case --
implemented in this commit -- for `background-blur-radius` to be renamed
to `background-blur`.
The remapping is comptime-known which lets us do some comptime
validation. The remap check isn't done unless no fields match which
means for well-formed config files, there's no overhead.
For future improvements:
- We should update our config help generator to note renamed fields.
- We could offer automatic migration of config files be rewriting them.
- We can enrich the value type with more metadata to help with
config gen or other tooling.
