Describe your proposed improvement and the problem it solves.

Typography rules for the French language state that a thin nonbreaking space should be used before the so-called "high punctuation characters", ":;!?" (more precisely, some rulesets, but not all, state that ":" should be preceded by a fullwidth space). See e.g. the TeX docs for babel-french (https://ctan.org/pkg/babel-french, search for "AutoSpacePunctuation") or https://en.wikipedia.org/wiki/Colon_(punctuation)#Spacing or (in French, sorry) https://fr.wikipedia.org/wiki/Ponctuation#Signes_de_ponctuation_occidentaux.

In TeX documents using babel-french, one can skip typing the space; babel automatically inserts it with the right width and non-breakingness (luatex is smart enough to not insert it if the following character is not a space, e.g. in C:\A\Windows\Path or in 10:55). Likewise, in French documents, Word automatically inserts a nonbreaking space before these punctuation signs when typed interactively, and if the space is already there it gets autoconverted to nonbreaking (I don't think it distinguishes between thin and full space, nor does it detect cases where the next character is not a space). However, loading a Word file without the spaces doesn't add spaces automatically where needed.

It would be useful if pandoc could detect this case, and, when emitting docx (or html, for that matter), inserts the right missing space.

Describe alternatives you've considered.

Manually inserting nonbreaking thin spaces would be slightly tedious.