feat: static targetWidths functionality

[ericdeansanchez]

The purpose of this PR is to extend targetWidths functionality to
users via a static class function. A static function has been used to
preserve the import syntax for UMD, CommonJS, and ES6 modules.

Previously, this function was named _generateTargetWidths which
required users to instantiate an ImgixClient object to use it. Now,
this function has been renamed and re-implemented as a static function.

Previously, the parameter order required widthTolerance, minWidth,
and then maxWidth. The order has been changed in accordance with
how common a particular argument is expected to be passed:

targetWidths(minWidth, maxWidth, widthTolerance, cache)

[commit-lint]

Features

• static targetWidths functionality (d26946a)

Contributors

ericdeansanchez

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[frederickfogerty]

Thanks so much for this @ericdeansanchez! Could we please update the TS definitions to include this change?