Add no_export attribute and export all enums by default

[RunDevelopment]

This adds the no_export attribute we talked about. Enums with the no_export attribute will not be exported in JS and TS. This makes them consistent with structs that are also exported by default and provides an option to opt out.

The no_export attribute is completely independent of the visibility of the Rust enums. E.g. an enum may be pub and non-exported, or private and exported. To make this consistent, string enums no longer use the enum's visibility to determine whether their type is exported.

Changes:

• Add no_export attribute.
• Changed behavior of private string enums. Their type will now be exported by default.
• Allow private numeric enums to be consistent with structs and string enums.
• Add no_export to all string enums in web-sys.

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TODO:

• Numeric enums with no_export that are used in exported function are currently typed as non-exported TS enums in TS. This makes the exported functions with such enums are parameters a lot less useful, since they can't be invoked without either (1) TS compiler errors or (2) a type cast to any to work around the type checker.
• Documentation. We may want to think about the details here a bit more. I'll write the docs when we have everything figured out.
• Think about how users should deal with third-party crates with string enums that are now exported by default.