Various code actions about types

[pluralia]

Write some code actions as quick fixes for validations from #426:

• [ ] type names duplication -> removes a duplicated type
• [x] validation of property name duplication (#430) -> removes a duplicated property (dupes are flagged as an error, but no action to remove)

More code actions:

• [ ] adds a declared type by inferred type name
• [ ] replaces a dummy rule with a declared type
• [x] adds return T to rule for general error The type 'T' is already explicitly declared and cannot be inferred. From #449.

[montymxb]

Added a third case to more code actions above.

[montymxb]

I can take this one :octocat:

[montymxb]

This one has been around for awhile. I would vote to close this out and create separate issues instead:

• generate declared type for rule (from inferred type)
• identify & recommend replacing a dummy rule with a declared type
• code action to remove unused/duplicated declared type (first of non-unique list)
• code action to remove unused/duplicated property (first of non-unique list)

The last two are akin to how TS support handles duplicate declarations, offering to remove the leading one of any number.