Fix RichTextLabel's modified stack being wiped on translation changes

[YeldhamDev]

Fixes #96937.

[bruvzg]

• set_effects() and install_effect() use

if ((!text.is_empty()) && use_bbcode) {
	parse_bbcode(atr(text));
}

instead of

if (!text.is_empty()) {
    _apply_translation()
}

and should be updated to take stack_externally_modified into account (unlike other cases, it's only updating is bbcode is enabled).

• remove_paragraph() should also set stack_externally_modified = true;.
• _apply_translation() can have early if (text.is_empty()) { return; } to avoid calling parsers.

[YeldhamDev]

@bruvzg Sorry for the wait, changes made.

[Repiteo]

Codestyle checks out

[Repiteo]

Thanks!