[requirement] support Microsoft webview2 fixed version mode

[congzhangzh]

Purpose:

deploy .NET core webview_csharp application as a whole and self-include a fixed version WebView2 runtime.

Trouble:

webView windows implement must allow to pass browserExecutableFolder to CreateCoreWebView2EnvironmentWithOptions. which is not possible now. the code 
ref:
1. https://github.com/webview/webview/blob/master/webview.h#L871
2. https://docs.microsoft.com/en-us/microsoft-edge/webview2/reference/win32/webview2-idl?view=webview2-1.0.622.22#createcorewebview2environmentwithoptions

Solution:

how about auto detect subdir like webview2 or some environment variable  like WEBVIEW_WEBVIEW2_INSTALL_DIR? which just need change the code near https://github.com/webview/webview/blob/master/webview.h#L871,   and no new api or webview browser abstract api change is not needed.

ref:

1. [requirement] support Microsoft webview2 fixed version mode webview_csharp#11
2. https://docs.microsoft.com/en-us/microsoft-edge/webview2/concepts/distribution#fixed-version-distribution-mode