Bootstrap/Load: Add `local` environment type to `wp_get_environment_type()`.

This gives developers a better control over their existing development workflow and ensures that `local` is not the exact same as `development` if it does not need to be.

Props claytoncollie, johnbillion, jeremyfelt, kreppar, dushakov, TimothyBlynJacobs, Ipstenu, khag7, knutsp, Clorith, markjaquith, joostdevalk, SergeyBiryukov.
Merges [48856] to the 5.5 branch.
Fixes #51064.
Built from https://develop.svn.wordpress.org/branches/5.5@48857


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48619 1a063a9b-81f0-0310-95a4-ce76da25c4cd