Fixes an issue identified in the upcoming QGIS Map Design 2nd ed
(spoiler alert!) where it's impossible to utilise label text
substitutions if you also want to use word wrapping.
This isn't possible to directly fix, because we need to evaluate
the full label expression (including the word wrapping component)
in order to actually HAVE text to substitute into.
So, a new setting has been added to the label formatting tab
allowing users to directly set an auto-wrapping line ideal
line size. This is applied AFTER label text evaluation, substitutions,
and the 'wrap text on' character, so it can play correctly well with
all these other settings. This also has the nice side-effect
of making auto label text wrapping more accessible to new
users/those unfamiliar with the wordwrap expression function.
Fixes#20007, and cleans up a chapter of QMD 2ed ;)
