idl: Move IDL types from the `anchor-syn` crate to the new IDL crate

[acheroncrypto]

Problem

IDL types currently live in anchor-syn crate, which is an internal crate that handles code parsing and generation.

With the introduction of the new IDL crate, it's more appropriate for the IDL types to be inside the new crate.

Summary of changes

• Move IDL types from anchor-syn crate to the new IDL crate
• Move the IdlBuild trait inside the new IDL crate
• Remove idl-types feature of anchor-syn

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