Add support for JsonManifestVersionStrategy

[dbu]

wrap up #1525

• Why is JsonManifestVersionStrategy not found by phpstan?
• Adjust Resources/doc/asset-versioning.rst to document the new functionality
• Check file format changes @wouterSkepp

[dbu]

@wouterSkepp could you please do a pull request against this branch to adjust the asset versioning documentation ? apart from that i think we are ready to merge.

[wouterSkepp]

@dbu I might have overlooked this bundles ability to change formats in my tests. (eg. the original file is a png and the filter reformats it to a .jpg)
I just stumbIed across #1198 about it; I've not tested this PR against these cases and I don't think it's handled correctly by my current implementation. (e.g. the filtered jpg still has it's .png extension)

I'm willing to have a look at this later on but it'll take a while because of vacationing plans.

[dbu]

thank you for the heads up! then i will refrain from merging until you (or somebody else) has been able to check that use case and possibly adjust this PR to handle it correctly.

[coveralls]

coverage: 81.749% (-0.2%) from 81.998% when pulling 6f13e00 on json_manifest_version_strategy_support into cb64b05 on 2.x.

[dbu]

@wouterSkepp do you have time to look into this so we can wrap up the feature?