fix(parse): correct quotes matching for TermExec commands

[NTBBloodbath]

Hey, after a while without using my TermExec commands I decided to trigger them and found that they weren't working properly since the command was null instead of the passed value (e.g. git status) when double quotes are used for enclosing the command. This PR aims to fix this behavior by checking two things before matching the command with the quoting.

1. Checks if the current OS is Windows or not by using jit.os.
2. Checks if the +shellslash feature exists and if it is turned on.

But, why check those things? Well, they're needed for some reasons:

1. The Vimscript shellescape function works in a very peculiar way regarding the quoting.

From the shellescape function docs:

On Windows when 'shellslash' is not set, encloses {string} in double-quotes and doubles all double-quotes within {string}.
Otherwise encloses {string} in single-quotes and replaces all "'" with "'\''".

2. The shellslash feature is only for Windows, that means we also need to check its value in addition to point 1 if the current OS is Windows because of how shellescape manages the quoting.

So basically, what was happening is that if you have used double quotes for enclosing your command the value:match(quotes) statement will return nil because the shellescape function enclosed the command in single quotes.

I only have tested in Linux because I don't have access to a Windows machine, but should work as expected too. Let me know if something needs to be changed.

[akinsho]

@NTBBloodbath thanks for raising this PR, been a bit busy but will take a look later on today. Just to clarify, is this issue you are seeing just on Windows or also on Linux?

[akinsho]

@NTBBloodbath just tried it out and noticed what you meant, thanks for picking this up and fixing it. I realised all my tests and usage favoured single quotes, so I didn't pick this up