KaraTemplater: Fold generated output

[petzku]

Requires users to be running a fold-supported Aegisub build for this to do anything, but works on non-folding builds too.

[petzku]

We could also consider requiring a "directive" somewhere in the file to set this, e.g. a commented line with directive folds, if it's not desired to fold output by default.

Said line could also define e.g. fold default state (expanded/collapsed), if we want to go down that road.

Possibly something like below (probably, generate=true would be the default if the directive exists, but generate=false could allow the directive to remain in-file despite being disabled):

Comment: 0,0:00:00.00,0:00:00.00,Default,,0,0,0,directive fold,generate = true, collapsed = false

[The0x539]

This needs to be opt-in by way of some form of directive feature, which would have to be implemented first (possibly starting as a means of customizing some other script-global behavior).

[petzku]

A possible way to do this before said directive feature exists could be setting a special variable, though that risks backwards incompatibility (if a user happens to incidentally use a variable by that name), and is probably not worth it.

What other directives might be useful?

[petzku]

I have been informed this PR errors when the template generates no lines. While this is a rare case, it nevertheless should be fixed.