Add textobject for current indentation

[EpocSquadron]

Kakoune has Alt-i i, which will select everything at the same indentation level as the existing selections, above and below until indentation level changes. Would be nice to have this in helix as well, as it's handy for quickly altering the indentation of a whole set of lines that might not be grouped under a single tree-sitter node.

[space-shell]

This enhancement would work really well for markup languages without tree-sitter availability such as .tsx files.