Make CharFXTransform.font property read only, add some extra documentation notes.

[bruvzg]

See #102333

In most cases, setting font value will result in incorrect rendering, since it's CharFX is applied after text shaping (and therefor changing font require replicating shaping process and in many cases completely impossible). font value was intended to get replacement glyph indices, not to change it.

[bruvzg]

If a way to fully replace text from custom FX is desired, it might be added as a dedicated type of FX which can do shaping on its own. But I'm not sure if there are many uses for it.

[Repiteo]

Thanks!

[Qubus0]

Aw, this was handy to both make more complex font changing effects and for custom icon fonts (which could easily be created as .fnt files). way nicer than always needing the full image or font path for [img] and [font]

granted this version mostly works with monospace fonts and has a few flaws for other spacings: godotengine/godot-proposals#3450 (comment) but i think all the methods necessary to retrieve the correct transforms are accessible to gdscript - i just haven't had the time to test my theory after that.

And yes there are quite a few good uses for replacing text with custom fx -> https://github.com/SimonGigant/godot-text_effects_4.0