fix(extmarks): adjust extmarks when inserting prompt prefix

Problem:    The 'statuscolumn' width is being estimated without the
            proper context. In particular, this resulted in the fact
            that a custom fold column could be included in the estimated
            `number_width()`, and doubly added when actually drawing the
            statuscolumn due to `win_col_off()` also adding the
            `'foldcolumn'` width. Resulting in a status column that is
            `'foldcolumn'` cells wider than necessary.
Solution:   Estimate 'statuscolumn' width in `get_statuscol_str()` when
            a buffer's line count has changed.

This prevents the TUI from doing unexpected things when receiving a
deadly signal or running out of memory.

fix(man): use italics for <bs>_

Even though underline is strictly what this should be. <bs>_ was used by
nroff to indicate italics which wasn't possible on old typewriters so
underline was used. Modern terminals now support italics so lets use
that now.

See:
- https://unix.stackexchange.com/questions/274658/purpose-of-ascii-text-with-overstriking-file-format/274795#274795
- https://cmd.inp.nsk.su/old/cmd2/manuals/unix/UNIX_Unleashed/ch08.htm

refactor(tests): run unittests using main nvim binary - delete separate nvim-test build

This allows us to get rid of the separate "nvim-test" target

fix(treesitter): validate language name

This fixes "fake server" from leaking memory, which makes ASAN very
upset, except on current ASAN CI for some reason.