CI: Build multi-platform paclet

[maxitg]

Changes

• Closes #552.
• Renames CI jobs to explicitly say if they do building or testing.
• Copies the libraries from macos-build and windows-build, and puts them into the paclet created in wolfram-language-paclet-test before testing.
• As a result, the paclet-artifact from wolfram-language-paclet-test now contains libraries for all platforms.
• Also updates contributing guide accordingly to describe the new workflow and remove references to the internal build process.

Examples

• Here is the workflow.

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