RichTextLabel: Add ability to get col/row position at mouse or arbitrary point

[me2beats]

Describe the project you are working on:
gdscript plugins

Describe the problem or limitation you are having in your project:
In gdscript I can't find a way to get col/row position at mouse for RichTextLabel.
maybe it would also be useful to have this feature for a regular Label

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
Adding appropriate api methods would solve the problem

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
get_position_at_mouse() returns the text column an row indices at mouse
or
get_position_at_point(point:Vector2) returns column an row indices at some point
Imo the second one is more flexible.

If this enhancement will not be used often, can it be worked around with a few lines of script?:
no it can't, since there's no api for that (available in gdscript)

Is there a reason why this should be core and not an add-on in the asset library?:
not sure