Add BBCode escape sequences for left/right brackets

[dalexeev]

This PR adds [lb] and [rb] BBCodes, which are displayed as [ and ] respectively. This solution is consistent with the existing ones [lrm], [rlm], etc. BBCodes for Unicode control characters.

[b]text[/b]	text
[lb]b[rb]text[lb]/b[rb]	[b]text[/b]

Resolves #56556.

[bruvzg]

All of these codes probably should be added to the https://github.com/godotengine/godot-docs/blob/master/tutorials/ui/bbcode_in_richtextlabel.rst, BBCodes for the special characters is not currently documented anywhere.

[akien-mga]

Seems fine to me. Another option would be to add support for \[ and \] to escape brackets, so this would become \[b\]text\[/b\].

That's maybe more common/expected at least now that Markdown is so popular, but might also be more difficult to implement, so I'm fine with using [lb] and [rb] for literal brackets.

[akien-mga]

This should maybe be added to doc/tools/make_rst.py too for the class reference XML+BBCode to RST converter. So far we've always enclosed non-tag brackets with [code] [/code] (which treats everything within as literals), and that seems to be good enough, but it probably makes sense to have this feature there too.

There are more of our BBCode tags which are supported in RichTextLabel but not in make_rst.py, we should probably document that for doc writers (but that's outside the scope of this PR).

[dalexeev]

All of these codes probably should be added to the https://github.com/godotengine/godot-docs/blob/master/tutorials/ui/bbcode_in_richtextlabel.rst, BBCodes for the special characters is not currently documented anywhere.

If this PR is accepted, then I will open the corresponding PR in the godot-docs repository.

BBCodes for the special characters is not currently documented anywhere

They are documented, just the preview on GitHub does not display them.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.5.