docs: clarify "pipe" mode for sockconnect

Problem:    FIRRTL files are not recognized.
Solution:   Add a pattern for FIRRTL files. (Amaan Qureshi, closes vim/vim#11931)

https://github.com/vim/vim/commit/685bf83b73d0fe6fd36bb2949bebd6aae66a139e

Co-authored-by: Amaan Qureshi <amaanq12@gmail.com>

Make it more clear that on unix the "pipe" mode of sockconnect uses unix
local domain sockets, not named pipes (FIFOs) which are not currently
supported.

See discussion in #22080.

Signed-off-by: Thayne McCombs

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

Problem:    .clangd and .stylelintrc files don't get a filetype.
Solution:   Use yaml for .clangd and json for .stylelintrc files. (Mark
            Skelton, closes vim/vim#11916)

https://github.com/vim/vim/commit/9c51798a1f3b79ace5ae0551a8bb122025ac94ed

Co-authored-by: Mark Skelton <mdskelton99@gmail.com>

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

fix(treesitter): validate language name