Add SPM support

[DwayneCoussement]

This PR adds basic SPM support.
Can be tested by integrating the master branch of the forked repo. One should be able to remove a cocoapod and use the Swift Package instead without any further code changes.

[DwayneCoussement]

@dreampiggy, could you review this one pls? 🙏

[dreampiggy]

Hi.
Recently I'm busying in daily work and may have no enough time for open-source project. I'll have a check for small changes PR soon.
For large refactory or features, please try to summerize the idea and core changes, and I may check back in weekend.

[dreampiggy]

This one seems need a little changes, please have a check like the one https://github.com/SDWebImage/SDWebImagePhotosPlugin/blob/master/SDWebImagePhotosPlugin/Classes/SDWebImagePhotosDefine.h

For example:

1. The @import code should use the #if __has_include <XXX/XXX.h> #import <XXX/XXX.h> #else @import XXX compatibility hack. Because this repo need to support CocoaPods user who don't use module.

[DwayneCoussement]

Thank you for the review; I've changed the imports and tested both against cocoapods and spm.

The only thing really added besides the imports is a Package.swift (and an autogenerated Package.resolved file).

[dreampiggy]

Does SVGKit supports watchOS ? The package.swift list the watchOS_v2 ...

The podspec and Xcode seems does not add watchOS target. I remember the time when I create this repo, SVGKit does not have any watchOS.

[DwayneCoussement]

Correct, I've updated the minimum platform requirements to align with SVGKit, great catch 👍

[dreampiggy]

LGTM now.

[dreampiggy]

Released v1.4.0

[DwayneCoussement]

Thank you 🙌