Publish Processing Gradle plugins

[Stefterv]

Add a publish-gradle job to the release workflow to publish Processing libraries to the Gradle Plugin Portal.

This way future processing libraries can be created by adding

plugins{
    id("org.processing.library") version "4.5.3"
}

processing {
    library {
        version = 1
        prettyVersion = "1.0.0"

        authors = mapOf(
            "The Processing Foundation" to "https://processing.org"
        )
        url = "https://processing.org/"
        categories = listOf("file", "exporter", "dxf")

        sentence = "DXF export library for Processing"
        paragraph =
            "This library allows you to export your Processing drawings as DXF files, which can be opened in CAD applications."

    }
}

To any standard Java/Kotlin gradle project, this will then handle packaging the library. We can add more functionality to it over time and the only thing library authors will need to do is update the plugin version.

TODO:

• Create Gradle Portal Account (account created with the processing-bot)
• Set repository secrets (GRADLE_PUBLISH_KEY, GRADLE_PUBLISH_SECRET)

After merge:

• Update the processing gradle library template

Related:
#1347
#1233 (needs extra step added to this PR)