As suggested in
WebAssembly#6181 (comment),
using `std::optional<Type>`, this unifies two different versions of
`make***`, for  block-like structures (`block`, `if`, `loop`, `try`, and
`try_table`) with and without a type parameter.

This also allows unifying of `finalize` methods, with and without a
type. This also sets `breakability` argument of `Block::finalize` to
`Unknown` so we can only have one `Block::finalize` that handles all
cases.

This also adds an optional `std::optional<Type> type` parameter to
`blockify`, `blockifyWithName`, and `makeSequence` functions in
`wasm-builder.h`.