emacs: do not highlight `#foo]` as an attribute

part of #14347
emacs: highlight `#[foo = "bar"]` attributes

Setting `OVERRIDE` to `t` overrides the conflicting highlight Emacs
places on string literals.

Fix #14347
auto merge of #14818 : tomjakubowski/rust/emacs-attributes-key-value, r=pnkfelix

This addresses two problems noted in #14347: the highlight of `#foo]` as an attribute, and the non-highlight of `#[foo = "bar"]`.