Follow up issues for #28587

[agocke]

There's some follow up work for #28587, which implements the first end-to-end compilation of the using var feature.

• Need to support multiple declarations
• Need to add binding check to see if the type can be used as a using
• Need to store information from the using binder so we know which method to call and which conversion to do
• Address diagnostic messages to be up to date:
• • ERR_NoConvToIAsyncDisp: async -> asynchronous, to avoid implying an async using (see Implement async pattern dispose #31201 (review))
• • ERR_NoConvToIAsyncDisp: seems incomplete (a public DisposeAsync() method works too) (see Implement async pattern dispose #31201 (review))
• • ERR_NoConvToIDisp: confirm whether this message should say "instance" (see Implement async pattern dispose #31201 (comment))
• • allow extension methods
• • use ERR_NoConvToIAsyncDispWrongAsync (added in Async-streams: minimal test for IOperation and CFG. Improve diagnostics #30363)
• Use CSharpTestBase.AsyncStreamsTypes instead of copying around the interface in tests