Allow custom templating for newly created files

[anderseknert]

Since we're now in the business of templating too 😄

When a new file is created, and we currently add:

package inferred.from.path

import rego.v1

It'd be kinda cool if we'd allow users to customize this with their own templates, in .regal/templates or whatever. So that for each new file created, you'd be able to get things like metadata annotations on the package, tests that import the package they're testing, the common library used everywhere, and whatever else we think could be useful. For templating, I'd imagine we'd provide some useful variables that they could inject, like the package name, bundle data, regal config and so on.

We should also make it so that it's possible to create files from template not just from the language server, but from something like the regal new command too.