Refactor: Consolidate Bash variable name initialization

> Specifically, we would need to change https://github.com/amber-lang/amber/blob/main/src/modules/variable/init.rs#L72, and other lines like it. This is one reason I've been wanting to centralise all Bash variable name generation, so we don't have to go searching for all instances... but that may be a larger task than is called for at this point.

_Originally posted by @hdwalters in [#666](https://github.com/amber-lang/amber/issues/666#issuecomment-2593524730)_