Support !yamlscript v0: ... meta section

[ingydotnet]

A YAMLScript file needs to declare up front many things that the compiler should be aware of.

The initial !yamlscript/v0/... tags let you control "ys version" and "starting mode" but there are many more things we need including:

• YAML Schema
• SchemaYS Schema
• YS version(s) required
• Capabilities
• Vars file(s) to import
• Libraries to use
• Namespace to use
• Dependency sources to use
• Experimental features to use

A YS document (must be a top level map) can have a first pair whose key is !yamlscript v0:.

For example:

!yamlscript v0:
  mode: data/code/bare
  yaml: 1.2 core
  scys: schemays-url
  need: 0.1.87
  with: capabilities-config
  vars: vars-url
  name: namespace
  uses: [module-urls]
  deps: dep-specs
  exps: [experimental features]

The values can also be stored outside in any importable form:

!yamlscript v0: ./meta.yaml

or

!yamlscript v0: github:user/repo/meta.yaml

NOTE: If the special key tag is used, the special document is not used. (one or the other)

[ingydotnet]

Top level sequences can be used like so:

!yamlscript v0:
  mode: data
  yaml: 1.2 json
:::
- foo
- bar