Extend variants for tree-sitter textobject motions

[the-mikedavis]

Tree-sitter based textobject motions (for example ]f/[f) do not currently have different implementations between normal mode and select mode (v).

The behavior of these motions in select mode is worth some discussion. I see two options:

1. Like ]p/[p or w/b in select mode, they should keep the anchor and move the head. This "extend" behavior is the normal behavior of motions in select mode. There is prior implementation-work for this behavior: Include treesitter objects in select mode #2802. (I'm looking at pulling this across the finish line now.)
2. Like n/N, they should add new selections containing the next/previous textobject.

Option 2 is a bit unconventional (n/N is the only other that uses this behavior off the top of my head) but I think it would be more useful. Tree-sitter textobjects may not be contiguous in a file in the same way paragraphs are (by ]p/[p's definitions), so I think v]f would select other text between two function definitions I would not want. Even if this is not selected as the correct behavior, it might be good to adding separate commands that implement this behavior.

[the-mikedavis]

On second thought, option 2 may be better served by #3007