Add VCPKG_TARGET_IS_EMSCRIPTEN, fix opus and libsodium ports for wasm32-emscripten

[past-due]

Describe the pull request

• vcpkg_common_definitions: Add VCPKG_TARGET_IS_EMSCRIPTEN
• [opus] Fix wasm32-emscripten builds
• [libsodium] Fix wasm32-emscripten builds

Which triplets are supported/not supported?

wasm32-emscripten

[github-actions]

You have modified or added at least one portfile where deprecated functions are used.

If you feel able to do so, please consider migrating them to the new functions:
vcpkg_install_cmake -> vcpkg_cmake_install (from port vcpkg-cmake)
vcpkg_build_cmake -> vcpkg_cmake_build (from port vcpkg-cmake)
vcpkg_configure_cmake -> vcpkg_cmake_configure (Please remove the option PREFER_NINJA) (from port vcpkg-cmake)
vcpkg_fixup_cmake_targets -> vcpkg_cmake_config_fixup (from port vcpkg-cmake-config)

In the ports that use the new function, you have to add the corresponding dependencies:

{
  "name": "vcpkg-cmake",
  "host": true
},
{
  "name": "vcpkg-cmake-config",
  "host": true
}

The following files are affected:

• ports/opus/portfile.cmake

You have modified or added at least one vcpkg.json where you should check the license field.

If you feel able to do so, please consider adding a "license" field to the following files:

• ports/opus/vcpkg.json

Valid values for the license field can be found in the documentation

[Adela0814]

Pinging @past-due for response. Is work still being done for this PR? Please fix the conflicts.

[past-due]

@Adela0814 Rebased and ready for merge