Bug 1624524 - Only compile wasmTextToBinary and wasmCodeOffsets in JS Shell. r=bbouvier

This commit moves the implementation of the wasmTextToBinary and wasmCodeOffets
functions to the JS shell.

Before this change, the implementation was in wasm/rust/. This commit moves the
implementation to js/rust, which is only compiled for the JS shell. The C++
bindings for the rust code is moved to the shell/ directory.

Differential Revision: https://phabricator.services.mozilla.com/D68650

--HG--
rename : js/src/wasm/rust/src/lib.rs => js/src/rust/wasm.rs
rename : js/src/wasm/WasmTesting.cpp => js/src/shell/WasmTesting.cpp
rename : js/src/wasm/WasmTesting.h => js/src/shell/WasmTesting.h
extra : moz-landing-system : lando