chore(common): define constants for file types

Now that we have the file type detection in place, we can replace
existing references to the file extensions, where possible, with the
constant declarations. While we could rely on TypeScript's typing to use
string-based values, this helps us to avoid using file extensions
directly in string transforms, regex, etc, and use the `KeymanFileTypes`
alias instead.

Note that only references in common/web, and kmc-* projects have been
touched. References in other Developer projects have not been touched,
as some of those are not yet ESM, so that needs to be addressed first.