WASM/Parser: refine MKField with a vector invariant

Context

Following !5965 (merged), we can refine the MKField state to only take a vector field on the type level. This has two good properties:

• it removes two assert false for impossible cases in the parser, and the invariant is now in the type itself;
• it avoids writing two impossible encodings

Also removes section_tag from the module continuation: it was redundant with field_type.

Manually testing the MR

make test-webassembly

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
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