Forward all user's parameters set in PAGER and improve flag detection

[kdeldycke]

Another PR following up on pager refactors that:

• Forwards all user's parameters set in PAGER
• Improve detection of raw mode by extending flag and option parsing
• Fix detection of less.exe on Windows (even if not currently exercised, this make the method future-proof)

But more importantly: adds dozen of testing with weird edge-cases around pager's internal methods.

Follows up on:

• Refactor pager stream handling #3767
• Resolve the pager command once, in _pager_contextmanager #3776

[davidism]

Seems right to me.

[kdeldycke]

Seems right to me.

Just a note re-reading my code, I am not 100% sure that I am covering all the funky edge cases that a user can put in a LESS environment, so I regrouped the parsing bits in a _less_uses_raw_mode private method. So in the future, we can easily update it and its paired test_pipepager_less_raw_mode_detection test to add more edge cases.