fix: Unescaping certain regex options.

[yjyao]

Many regexes contain escaped characters
like \s or \w.
tmux-fastcopy queries the tmux environment
for regex overwrites.
Specifically,
it uses the tmux show-options -g command.
The output in turn escapes the backward slashes,
turning \s into \\s.
To correctly parse the regex overwrites,
tmux-fastcopy needs to unescape these backward slashes too.

The tmux show-options -g command prints values in three forms:

• Unquoted for simple, unspaced values:

  $ tmux set @myopt hey ; tmux show @myopt
  @myopt hey

• Single-quoted for simple, double-quoted values:

  $ tmux set @myopt hey\"world ; tmux show @myopt
  @myopt 'hey"world'

• Double-quoted for other values:

  $ tmux set @myopt hey\  ; tmux show @myopt
  @myopt "hey "

Note that the output escapes backward slashes
in all three forms:

$ tmux set @myopt '\hey' ; tmux show @myopt
@myopt \\hey

$ tmux set @myopt '\"hey' ; tmux show @myopt
@myopt '\\"hey'

$ tmux set @myopt '\ hey' ; tmux show @myopt
@myopt "\\ hey"

For many regexes,
tmux would print the value unquoted.
For example, the path regex built-in to tmux-fastcopy is like this:

$ tmux set-option @myopt '(\b([\w.-]+|~)?(/[\w.-]+)+\b)' ; tmux show @myopt
@myopt (\\b([\\w.-]+|~)?(/[\\w.-]+)+\\b)

Currently,
tmux-fastcopy relies on strconv.Unquote
in the tmuxopt.Loader.ReadValue function
to unescape the backslashes.
However,
a caveat is that strconv.Unquote only performs
if the given string is quoted.
As a result,
tmux-fastcopy never unescapes the regex from the last paragraph, and would fail to match it.

This commit fixes the problem
by manually double-quoting unquoted strings,
forcing strconv.Unquote to unescape them.

(The above results were tested with tmux 3.3a in bash 5.2.15(1)-release.)

---

TIP: Before this PR is merged (if ever),
a workaround is to force tmux to double-quote the value
for example by prefixing your regex with a "{0}
which matches nothing.

[abhinav]

This is excellent! Thanks for the fix, and the thorough explanation. Much appreciated!
I added a small integration test to verify this behavior with running copies of tmux, and that appears to be green for all versions of tmux we're testing against.

[abhinav]

Released in 0.14.1. Thanks!