Linked javascript files

They are emcc way how to extend the dotnet.native.js script during linking, by appending the scripts. See https://emscripten.org/docs/tools_reference/emcc.html#emcc-pre-js

There are -extern-pre-js,-pre-js, -post-js, -extern-post-js. In src\mono\browser\build\BrowserWasmApp.targets we apply them by file naming convention as: *.extpre.js,*.pre.js, *.post.js, *.extpost.js

In src\mono\browser\runtime\CMakeLists.txt which links only in-tree, we use same mapping explicitly.