input: arguments quoting: support single-quotes

Users expect single quotes to work when the value includes literal
backslashes or double-quotes (or as general quoting like in shell).

The updated docs also include some previously-missing notes:
- newline is only supported in double quotes.
- adjacent (quoted) arguments don't join into one.

Supporting mixed quoting (adjacent quoted strings) would make
mpv's parsing more complete, but would require delicate effort of
larger scope, for two reasons:
- We'd need to also support escaping outside of quotes and do our
  best about backward compatibility.
- The parsed value can either be a substring of the input or
  a newly-allocated string, which would be delicate when joining.

Not critical to add right now.

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