Improve native format (.ga) schema

[davelopez]

With #18, #19, and #20, we can now get a great deal of functionality out of the native .ga format schema definition but the schema is largely incomplete and inaccurate (initial version generated by an existing sample workflow).

How to improve it

The schema is easy to extend and improve, in addition to the standard JSON schema properties defined in the documentation, we can use some custom properties supported in VSCode to use Markdown formatting in hovers or insert snippets.

Parts that need improving

• Document existing properties using the markdownDescription attribute.
• Add missing properties
• Add correct properties to required and leave out the optional ones
• Fix incorrect type assignments to properties
• Add default values or possible enum values to properties with known values
• Add defaultSnippets where auto-completion can be beneficial