refactor: switch to `.bazelisk-fragment`

`.bazelisk-script` implies everything must be a script reference.

We may want to include other references in the future.

Fragment is better nomenclature here.

BREAKING CHANGE: `.bazelisk-script` is now `.bazelisk-fragment`

Update any `!reference[.bazelisk-script, ...]` to `!reference[.bazelisk-fragment, ...]`