[scripting] Port ScriptedEffects to QJSEngine

Summary:
QScriptEngine is deprecated for years and suffers bitrot.
Plasma hit one super major bug with it in 5.11.0 and has now ported
away.

Main porting notes:
- creating low level functions no longer exists
The old global functions are exposed on the ScriptedEffect instance
and then the QJSValue wrappers of the globalObject are modified to
trampoline the methods at a wrapper level.

- We can then use QJSEngine to automatically do argument error checking
rather than unmarshalling a QJSValue manually which significantly
reduces a lot of code.

- We can't make FPX2 a native type, so these are QJSValue args and
unboxed there.

Long term I want overloads for animate that take int/QSize/QPoint which
are native JS types, but that might be an API break.

Test Plan:
Hopefully comprehensive unit test which passes
Tested fade/fadeDesktop manually.

It's a very invasive change, so I expect some things will be broke
please help test any JS effects.

Reviewers: #kwin, mart, fvogt

Subscribers: fvogt, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14536