[Wasm] JS modularization (#61313)

[Wasm] JS modularization (#61313)

Modularized dotnet.js
* By using emcc options MODULARIZE and EXPORT_ES6, depending on WasmEnableES6 to produce ES6 or CommonJS modules respectively.
* WasmEnableES6 enables WasmBuildNative because dotnet.js need to be re-linked
* CommonJS version is able to be loaded into global namespace and behaves as before this change.
* Added new es6/*.js and es6/*.js files which are included in dotnet.js creation.
* Key aspects are documented in src/mono/wasm/runtime/modularize-dotnet.md of this PR.
* Improved dotnet.d.ts it is now generated into version control too, so that we could observe how it evolves.
* Removed legacy --testing argument and simplified is_testing logic in the the samples.
* Added browser-es6 sample app, which uses WasmEnableES6=true to compile dotnet.js as ES6 module.
* Added browser-legacy sample, which uses dotnet.js CommonJS module and loads it into global namespace.
* Added package.json into the nupkg

Co-authored-by: Ankit Jain <radical@gmail.com>
Co-authored-by: Katelyn Gadd <kg@luminance.org>
Co-authored-by: Marek Fišera <mara@neptuo.com>