Support `@type` and `@import-type` type aliases

[bmewburn]

• [x] @type
• [x] @import-type
• [x] Type substitution
• [ ] References . Refs for name, alias, from class, and type definition .
• [ ] Completion . Suggestions for imported type, from class, type definition, alias/name as type in other annotations.
• [ ] Rename. Import alias and type itself.
• [ ] Go to definition.
• [ ] optimisations
• [x] recursive substitution