Fix improper string escaping

[cpiber]

Before, input strings like '\\' or "\\" were improperly escaped. For double quotes, backslashes were not escaped at all and for single quotes, they were basically ignored (due to /\\?'/).

The above examples now produce '\\'/"\\" instead of '\'/"\" and are thus valid strings again.

[sindresorhus]

This will need some tests.

[cpiber]

So the change is desirable? Because it breaks existing tests that obviously relied on a "wrong" escaping... In that case I'll update that and write new tests :)

[sindresorhus]

So the change is desirable?

Yes

[sindresorhus]

Thanks :)