fix(out): escape backslashes in output shell scripts

[Ryooooooga]

abst: This is a suggestion that the \ contained in the output of pmy main should be escaped.

Currently, backslashes in the output of pmy main are not escaped.
For example, ~/.pmy/rules/pmy_rules.yml is:

- description: test
  regexp-left: ^(?P<query>.*)$ # <query> matches whole input
  cmd-groups:
    - stmt: print 'A\\nB' # print 'A\\nB' should output `A\nB'
      after: cat
  fuzzy-finder-cmd: fzf --query='<query>'
  buffer-left: '<query>'
  buffer-right: '[]'

Then, pmy main outputs the script below:

$ pmy main --buffer-left="" --buffer-right=""
local __pmy_out_command=$'print \'A\\nB\' ;'; ...

This means $__pmy_out_command == print 'A\nB' ;, and eval $__pmy_out_command outputs A<LF>B.

And echo also causes strange behavior.
The escape processing of the echo argument changes depending on the environment.
In the case of zsh, echo performs escaping, but command echo does not.

echo "--sh"
sh -c 'echo "A\nB"'
sh -c 'command echo "A\nB"'

echo "--bash"
bash -c 'echo "A\nB"'
bash -c 'command echo "A\nB"'

echo "--zsh"
zsh -c 'echo "A\nB"'
zsh -c 'command echo "A\nB"'

# Results in Alpine (Docker)
--sh
A\nB
A\nB
--bash
A\nB
A\nB
--zsh
A
B
A\nB

# Results in Ubuntu (Docker)
--sh
A
B
A
B
--bash
A\nB
A\nB
--zsh
A
B
A\nB

# Results in macOS
--sh
A
B
A
B
--bash
A\nB
A\nB
--zsh
A
B
A\nB

[relastle]

And echo also causes strange behavior.
The escape processing of the echo argument changes depending on the environment.
In the case of zsh, echo performs escaping, but command echo does not.

This is interesting knowledge I've never known.
Thank you for fixing!