Add compatible release (~=) syntax highlighting

[alexcanepa]

Another possible format for release specifications, compatible releases, uses a tilde to indicate that versions are allowed up to the next major release. For example, tensorflow~=1.12 is equivalent to tensorflow>=1.12, <2.0.

I haven't fully explored this package, but it seems like a good first step would be to add |~= to the below line: https://github.com/wuub/requirementstxt/blob/775e6dd62aa16f7b28632a200882ff6ad0315b01/requirementstxt.tmLanguage.yaml#L11