commenting: add 'inner comment' text object

[yochem]

Problem

The builtin default commenting feature also ads the gc text object, so e.g. dgc allows to delete the comment block. It would also be useful if there is an 'inner comment' to delete everything but the commentstring.

Currently, this can also be done using nvim-treesitter-textobjects with the @comment.inner capture, although that works slightly different, as it does not grab the 'largest contiguous block of non-blank commented lines around the cursor'.

It's something I, and I expect others too, use quite often to rewrite a whole comment. It would be nice to not depend on nvim-treesitter-textobjects for this (small) functionality.

Expected behavior

Example:

-- this is a comment
-- block

cigc:

-- |

Where | marks the cursor position.

Or a begin-end commentstring like HTML:

<!-- html comment -->

cigc:

<!-- | -->