fix: parse lines with multiple equal signs

[techouse]

In v0.5.4 I introduced a simpler way of splitting a line by =

envied/packages/envied_generator/lib/src/parser.dart

Line 42 in e089d00

final [String lhs, String rhs] = stripped.split('=');

However, this is inadequate when attempting to parse something like

SOME_URL=https://www.my-awesome-website.com/index.php?foo=bar&baz=qux

This PR fixes this by implementing String.partialSplit inspired by this StackOverflow answer

final [String lhs, String rhs] = stripped.partialSplit('=', 2);

If splitting the string would result in more than 2 items, the final element will contain the unsplit remainder of the string.

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Fixes #99

[techouse]

@petercinibulk once you have time, please throw an eye on this 😇