feat: introduce custom srcsets

[ericdeansanchez]

This PR implements custom srcsets by extending the parameter lists of
GenerateTargetWidths, BuildSrcSet, GenerateSrcSetDPR, and
GenerateSrcSetPairs.

First, target widths functionality was extended to support beginning
and ending at user-specified target widths where each subsequent
width increases within the specified tolerance. This was done to
prepare GenerateSrcSetPairs to accept a custom list of target widths.

Finally, GenerateSrcSetDPR has been extended to accept the
disableVariableQuality flag. When true, varied image output
quality is disabled. Passing "q" without disabling variable quality
exhibits the same behavior as passing "q" and disabling variable
quality.

During the development of this feature we decided that
SrcSetWidthTolerance should be a float i.a.w. our
imgix-core-js implementation.

The last portions implemented here is validation. We validate that
begin, end, and tol represent a valid srcset width range. We
also validate that any custom list of widths is not empty, null,
or does not contain negative "width" values.

[sherwinski]

Overall, this looks good to me. I just want to pose some questions about how we could possibly make this experience better:

[sherwinski]

This is almost there, just some final nitpicks and questions.

[sherwinski]

Wooo 👍