Add plugin schema

[bentsherman]

This PR refactors the repo to support multiple schema definitions, and adds a new schema for plugin specs

See also: nextflow-io/nextflow#6361

[bentsherman]

The terminology we are using in Nextflow is:

• A plugin spec is a JSON document that describes the language extensions of a Nextflow plugin
• The plugin schema is the JSON schema for plugin specs

I have proposed in this PR that we adopt similar language for params:

• A pipeline input spec is a JSON document that describes the parameters of a Nextflow pipeline
• The pipeline input schema is the JSON schema for pipeline input specs

In the future we can follow this pattern for pipeline outputs, samplesheets, and nf-core modules

[ewels]

Looks good to me!