disallow unrecognized escapes in unescape_string (part of #21284)

[digital-carver]

Throw an error when encountering unknown/unrecognized escape sequences in unescape_string (currently unrecognized single letter sequences like \m, \j, etc. are silently replaced by their corresponding character (m, j, ...) by unescape_string).

This seems to have been the intention behind test changes in #22800, and it will make fixing things like #27125 easier.

[digital-carver]

Added tests for the new changes. Realized a bug in the original commit regarding \ and " characters, fixed it.

In adding further tests to catch such bugs in the future, came across a pre-existing test (@test repr(ch) == "'$(isprint(ch) ? ch : st)'") that would fail for the \ character (afaict it should have been failing anyway, except that it wasn't being tested with \ as ch). Fixed that by changing it to @test repr(ch) == "'$(isprint(ch) && ch != '\\' ? ch : st)'", but I'm not sure if that's the right way to go about it or if it's too ad-hoc. In any case, the tests successfully pass (locally) with these changes.