Fix #757 indentation of "\in"

[Matafou]

The lexer now glues a "" to an immediately following word if it is itself preceded by a space.

Note that for really good indentation you should try to add something like this to your configuration:

(setq coq-smie-user-tokens '(("\in" . "=")))

to tell the indentation grammar that \in has the same precedence as "=".

[proux01]

As noted by @andres-erbsen in https://github.com/coq/coq/pull/19530#issuecomment-2368048196 this was accidentaly targetting the wrong branch, fixed.

[proux01]

Upstream PR merged, please merge

[ppedrot]

I'm taking the liberty to merge this so as to fix the Coq CI quickly.