bind get_total_visible_rows function to gdscript

[vmjcv]

i need bind get_total_visible_rows function to gdscript
this is my problem(https://godotforums.org/discussion/comment/42260#Comment_42260)
when i bind the function in gdscript,everything is ok
I feel get_total_visible_rows is a required method, like get_line_count

[KoBeWi]

Label and RichTextLabel expose similar function, so it makes sense to include it here too. But any newly exposed method needs a documentation written in the PR (you can use doctool to generate blank entry and fill the description).

Also, up to discussion, but the equivalent method in Label/RichTextLabel is called get_visible_line_count(). Maybe here it should have this name too, for consistency? We don't need to rename the method, the binding could have a different name.

[akien-mga]

Superseded by #48528. Thanks for the contribution nevertheless!