Bidi capability

[Avid Seeker]

Currently, for a program like Vim to query the terminal support for Bidi text,
it goes through the workaround of setting env var $TERMBIDI. 

See: https://github.com/vim/vim/issues/5023

I'm suggesting the inclusion of the various Bidi capabilities listed by the
Freedesktop "BiDi in Terminal Emulators" proposal:

https://terminal-wg.pages.freedesktop.org/bidi/recommendation/

E.g: in "The escape sequences" section, a terminal can have support for:

* Implicit vs explicit
* LTR vs RTL
* Box mirroring or no box mirroring
* Autodetection or no autodetection
* Arrow keys swapping or no arrow keys swapping
* Combining characters support (unsure about this. Could be left to programs)
* Letter shaping

-- Avid Seeker

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