The name was a relict from the very first day of project structuring,
but this name here makes for much much nicer tab-completing to source
file names...

Also a logic change: The Number panel does not allow free text
anymore, i. e. if something is entered it _must_ be an expression (at
least for the warning to go away).
An exception are plain numbers, also possible in scientific notation.

I. e.:
2+2 <-- not ok
text <-- not ok
${{2+2}} <-- OK
${env:VAR} <-- OK
123 <-- OK
10e-3 <-- OK
(empty) <-- OK

Implement mechanism to translate custom syntax to tinyexpr before styling (so e. g. hardcoded substitutions are allowed exactly like in some filters)

Since we now do not "allow" free text anymore and request SLF expression syntax, a few other things will need to be considered (but in the end it'll be tied together more tightly).

The static function felt like bad design ever since the Textfield
started using it...