idl: Fix using constant identifiers as generic arguments

[acheroncrypto]

Problem

Using constant identifiers as generic arguments results in a compile error during IDL generation as explained in #3520.

This is because constant identifiers are parsed as GenericArgument::Type (rather than GenericArgument::Const) in the syn library, as they are indistinguishable syntactically:

https://github.com/dtolnay/syn/blob/bfa790b8e445dc67b7ab94d75adb1a92d6296c9a/src/path.rs#L113-L114

Summary of changes

Fix being unable to use constant identifiers as generic arguments by parsing generic arguments that look like a constant identifier as IdlGenericArg::Const rather than IdlGenericArg::Type.

Currently this means we check if the identifier is longer than 1 letter and is fully uppercase. It's not perfect, but it should work as expected if the user follows Rust's naming conventions.

Fixes #3520

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