Closes #ISSUE

Moves the settings content definitions into their own crate, so that
they are compiled+cached separately from settings, primarily to avoid
recompiles due to changes in gpui. In that vain many gpui types such as
font weight/features, and `SharedString` were replaced in the content
crate, either with `*Content` types for font/modifier things, or
`String`/`Arc<str>` for `SharedString`. To make the conversions easy a
new trait method in the settings crate named `IntoGpui::into_gpui`
allows for `into()` like conversions to the gpui types in
`from_settings` impls.

Release Notes:

- N/A

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Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>