lang: Add a sanity check for unimplemented token extensions

[acheroncrypto]

Problem

Codegen currently skips unimplemented token extensions:

anchor/lang/syn/src/codegen/accounts/constraints.rs

Line 940 in 79d1cec

_ => {} // do nothing

This is not ideal since extensions are defined internally by us (from the user's constraints). If a token extension constraint is specified, and we've included the extension in the internal extensions variable, then not doing anything with it later on should be considered an internal bug. Therefore, it's better to panic in this case rather than doing nothing.

Summary of changes

Panic via unimplemented! macro if an extension has a missing implementation.

[vercel]

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