fix(task): preserve trailing backslashes in task arguments (#34505)

Running `deno task <name> <args...>` failed with an "Expected closing
double quote" parse error when an argument ended in a backslash, which
is a common shape for Windows paths like `.\dist\`. The argv wrapper in
`get_script_with_args` double-quoted each argument and only escaped `"`,
`$`, and `` ` ``, so a trailing `\` ended up escaping the closing quote.
The task shell's double-quoted-string scanner also accumulates its
`was_escape` flag across consecutive backslashes, so there is no way to
safely terminate a double-quoted string ending in backslashes from the
caller side.

Wrap each argument in single quotes instead. Single-quoted strings in
`deno_task_shell` preserve their contents literally, so backslashes,
dollar signs, and backticks all pass through without interpretation.
Embedded single quotes use the POSIX `'"'"'` idiom (close, double-quoted
`'`, reopen).

Fixes #31453