[Parser] Parse wast scripts

[tlively]

The spec tests use an extension of the standard text format that includes various commands and assertions used to test WebAssembly implementations. Add a utility to parse this extended WebAssembly script format and use it in wasm-shell to check that it parses our spec tests without error. Fix a few errors the new parser found in our spec tests.

A future PR will rewrite wasm-shell to interpret the results of the new parser, but for now to keep the diff smaller, do not do anything with the new parser except check for errors.

[tlively]

• #6581 👈
• main

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