Finalize workflow output definition

[bentsherman]

Close #5103

TODO:

language improvements:

• support dynamic path
• move publish options to config
• remove publish section from process definition
• preserve preview version ?

runtime improvements:

• prevent leading/trailing slashes
• report undefined target names
• detect file collisions at runtime ?
• detect outputs that aren't published or used downstream ?

new features (might be handled separately):

• support JSON index file
• generate output schema
• include publish targets in inspect command ?

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