Add diagnostic for inconsistent string quoting styles

[emmericp]

No description provided.

[CppCXY]

Why not just use codestyle-check, to keep a consistent style of quotes throughout the project?
Additionally, you did not consider the situation of '"aa"', where diagnostics should not appear

[emmericp]

Updated to not count strings that contain the other quote inside the literal, good idea!

You are probably right that this probably doesn't belong here but in codestyle-check. The main reason is that codestyle-check wants a project-level configuration to setup this, but if you work on individual scripts or files and I want to keep these consistent without additional configuration. The example that motiviated me to do this is actually the LuaLS repo itself, it doesn't setup a quote style for codestyle-check and I kept using the wrong quotes without anything warning me.

And finally I mainly did this as an exercise to understand how diagnostics work, so I'm not sad if this doesn't end up getting merged.

[ari-party]

This would still be nice.

[sumneko]

Thank you for your PR! As mentioned earlier, this should be the job of codestyle-check. Your specific file diagnostic needs can be discussed separately.