Add support for disabling spell check

[SimonBrandner]

Requires matrix-org/matrix-react-sdk#8604
Requires element-hq/element-desktop#379
Notes: none

---

This change has no change notes, so will not be included in the changelog.

[turt2live]

this needs a bit of context - there's claims on the linked issues that we can't reasonably manage spell check for mac OS, hence the check.

Evidence to the contrary would be nice :)

[SimonBrandner]

this needs a bit of context - there's claims on the linked issues that we can't reasonably manage spell check for mac OS, hence the check.

Evidence to the contrary would be nice :)

The method had a rather unideal name :/

[turt2live]

bonus points if all the functions here get their public/private/protected declarations

[SimonBrandner]

#22235 :P

[t3chguy]

LGTM otherwise