Add (simple) support for quoted multiline variables in .env files

[MidasLamb]

This re-uses almost all previously existing logic, it only changes what constitutes a "line" when parsing the file.

This is not a fully fledged parser (e.g. escaping quotes doesn't work) but it should handle some additional cases now.

Fixes #965

[felixfontein]

Thanks for your contribution!

Please note that this is a breaking change and cannot be merged. There has been an earlier attempt at this (#622) that had to be reverted because of this (#706).

Ref: #1435

[MidasLamb]

Thanks for the quick response @felixfontein !

I quickly went through the issue, and I'd like to point out that this PR does not add a full dotenv parser.
On the encryption side, it just looks make sure that it considers an entire key-value pair, not just lines, but how to value is encrypted has not changed.
On the decryption side there is maybe a small inconsistency, that shouldn't be an actual issue, where it might turn a "\n" combination into and actual newline.
i.e.

FOO="bar\nbaz"

becomes

FOO="bar
baz"

after an encryption->decryption cycle. But semantically, these values are actually the same, so this shouldn't be a breaking change.

This PR is just meant as a small incremental step to allow for multiline strings, without changing anything else really. In my reasoning the changes introduced here just expand the amount of allowed values, without doing any actual internal changes.

The main issue seemed to be with this change: https://github.com/getsops/sops/pull/622/files#diff-c9aa3c7e333fa3bc6fa314822b8da1e449389f6bb14b68608e47d048c4997a08R104, where it adds single quotes unconditionally to values.

If you want some additional tests/proof about this addition not being a breaking change in the way the earlier code was (which seems more like a proper dotenv parser, while this is just a slight expansion upon the currently working code.)