settings: Introduce PRESERVED_KEYS to write default values (#15474)

This adds the optional `PRESERVED_KEYS` constant to the `Settings`
trait,
which allows users of the trait to specify which keys should be written
to
the settings file, even if their current value matches the default
value.

That's useful for tagged settings that have, for example, a `"version"`
field
that should always be present in the user settings file, so we can then
reparse
the user settings based on the version.

Co-Authored-By: Thorsten <thorsten@zed.dev>

Release Notes:

- N/A

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Co-authored-by: Thorsten <thorsten@zed.dev>