Allow setting locale in frontmatter

**Is your feature request related to a problem? Please describe.**
I am using astro's dynamic routing to build up my urls, and it would be really nice if I could add a hidden `locale` field to my collection so that it appears in the frontmatter. As it stands, it seems the only way to determine a file's locale is from the path/filename, unless I'm missing another option.

**Describe the solution you'd like**
Ideally, this would be as easy as adding a field like `{ name: "locale", label: "Locale", widget: "hidden", default: "{{locale}}", i18n: true},`

**Describe alternatives you've considered**
I tried to create a `preSave` handler, but it seems to only run once per entry, and expects `data` to be returned, not the i18n map which contains the data for the localized versions.