resetting font-ligatures means passing `nil` as the second argument
of `(set-font-ligatures!)`
Using `ligatures-ignored-major-modes` to cancel ligatures is too
brittle, because "resetting ligatures for `prog-mode`" would have bad
semantics with all its derived modes. The user probably just wants to
remove the default ligatures and then configure `foo-mode`, but pushing
`prog-mode` to `ignored-major-modes` might just disable ligatures across
all modes _derived from_ `prog-mode`.
This commit changes things in 2 ways:
- resetting ligatures now directly manipulates ligature.el internal
state (the `ligature-composition-table` alist)
- in order to work, Doom must maintain an extra invariant on that alist,
multi-modes keys (`'(foo-mode bar-mode)`), cannot be used, only single
modes.
That mostly means that users should _not_ use
`ligature-set-ligatures` themselves in private config, but instead
always rely on `set-font-ligatures!` which does splicing behind
curtains. Failing to do so would be mostly harmless though (it would
just make "resetting ligatures" only partially remove set ligatures).
Fix: #7433