emacs: migrate build-support/emacs to applications/editors/emacs/build-support

As a consequence of restrictions imposed by RFC 140 - Simple Package Paths [1]
-, files related to a package should be confined on the package directory.

Certainly this restriction does not apply to packages outside by-name hierarchy.

Nonetheless, this is an interesting organization heuristics: things that affect
Emacs should be confined inside Emacs directory. Besides a future migration, the
"debuggability" of a framework is way more enhanced when we know how to find all
its files.

A similar task was done before, when RFC 140 was not a thging yet - namely, the
migration of emacs-modes to elisp-packages [2].

[1] https://github.com/NixOS/rfcs/pull/140
[2] https://github.com/NixOS/nixpkgs/pull/123859