Eliminate symbols TRUE and FALSE from the ScriptFu language

There is no need for FALSE and TRUE symbols defined in the ScriptFu language. They are not in the Scheme language. Instead, the Scheme language has symbols #f and #t

If eliminated, the ScriptFu language is simpler and smaller.

Scripts are shorter.

Many scripts in the GIMP repo will need simple substitutions and transformations. Typically, a few lines per script. For example:

(if (= shadow TRUE) ...
=>
(if shadow ...

Related to #8340 (closed) and #8404.

See the draft MR which I will soon submit. It will have a document "Changes in ScriptFu for authors" with more explanation.

As often discussed, we don't have to do this, but if we do, now is the time, for GIMP 3. If you have an opinion or see any technical difficulties, please comment.