[llvm-objdump][Wasm][NFC] Create WasmDump.h

Summary:
Continuing from D77285, the external interfaces implemented by
`WasmDump.cpp` are now declared in `WasmDump.h` and moved into the
`llvm::objdump` namespace.

Reviewers: jhenderson, MaskRay, DiggerLin, jasonliu, daltenty

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D77990