Better detection of OSC 1337 support

[Kid]

Currently viuer dumbly checks iTerm in TERM_PROGRAM. However there are a handful of terminal emulators that support the OSC 1337 escape codes, i.e. iTerm2's image protocols:

• iTerm2 (of course)
• mintty
• WezTerm
• MacTerm
• SwiftTerm
• Mlterm
• hterm (though this one runs on Chromium OS/web, so probably viu doesn't need to care about it now)

There should be a better method of checking support of the protocol.

A workaround is just to fake iTerm to viuer.

[Atanas Yankov]

The list is really helpful, thank you! I am struggling to find information about SwiftTerm and MacTerm, though. Do they set either $TERM or $TERM_PROGRAM?

Mlterm & hterm look pretty niche to me, I'd rather not include them unless specifically requested.

[Atanas Yankov]

Added the first two in 42cd38c