Fix #157: Correct single quote escaping for POSIX shells

[talk2drys]

Fixed an issue with run_command("echo 'Hello'") where wrapping commands in args in single quotes caused invalid parsing in POSIX-compliant shells. In these shells, single quotes enclose literal text, making escape sequences like \' invalid. For more details, refer to the GNU Bash manual: https://www.gnu.org/software/bash/manual/html_node/Single-Quotes.html.

For example, calling run_command("echo 'Hello'") would be incorrectly escaped by vim.fn.shellescape as echo '\''Hello'\'', leading to invalid parsing. The fix replaces the outer double quotes with single quotes where necessary and command arg with double qoute, ensuring proper escaping and compatibility across different shell environments.