[spec] Update align and offset type in text format for memory64

[keithw]

In memory64/wasm-3.0,

• in the abstract syntax and binary format, the type of a memarg offset was updated to u64, and
• in the reference interpreter's text parser, the type of a memarg align $a = 2^n$ was updated to u64.

This updates the text format spec to match and adds tests for extreme cases. These surface some bugs/out-of-date parsing logic in WABT and wasm-tools.