Make StorageFile.GetFileFromPathAsync easier to use

[riverar]

Related to #8

StorageFile.GetFileFromPathAsync is a bit painful to use. I'm thinking of the following quality of life changes, thoughts welcome:

• Remove absolute path restriction. (PR apps don't broker their file I/O, this restriction seems a bit silly.)
• Fix inability to accept paths with forward slashes. (Windows supports both / and \ as a path separator, so it's not clear why this API is so rigid.)
• Fix inability to accept long paths (\\?\)

[jonwis]

@smaillet-ms - anything you'd like to add here? @riverar we don't have a Project Reunion based StorageFile implementation (yet!) but I'll file some internal issues to track improving the in-box one.

[riverar]

@jonwis Thanks. Some Reunion APIs, like XamlBinaryWriter.Write, require IRandomAccessStream, which makes things difficult.

Quick search through Reunion winmds:

findstr /s /i /m Windows.Storage *

Microsoft.UI\Microsoft\UI\Xaml\Markup\IXamlBinaryWriterStatics
Microsoft.UI\Microsoft\UI\Xaml\Markup\XamlBinaryWriter
Microsoft.UI\Microsoft\UI\Xaml\Media\ILoadedImageSurfaceStatics
Microsoft.UI\Microsoft\UI\Xaml\Media\Imaging\BitmapSource
Microsoft.UI\Microsoft\UI\Xaml\Media\Imaging\IBitmapSource
Microsoft.UI\Microsoft\UI\Xaml\Media\Imaging\IRenderTargetBitmap
Microsoft.UI\Microsoft\UI\Xaml\Media\Imaging\ISvgImageSource
Microsoft.UI\Microsoft\UI\Xaml\Media\Imaging\IWriteableBitmap
Microsoft.UI\Microsoft\UI\Xaml\Media\Imaging\RenderTargetBitmap
Microsoft.UI\Microsoft\UI\Xaml\Media\Imaging\SvgImageSource
Microsoft.UI\Microsoft\UI\Xaml\Media\Imaging\WriteableBitmap
Microsoft.UI\Microsoft\UI\Xaml\Media\LoadedImageSurface
Microsoft.UI.Text\Microsoft\UI\Text\ITextDocument
Microsoft.UI.Text\Microsoft\UI\Text\ITextRange
Microsoft.UI.Text\Microsoft\UI\Text\RichEditTextDocument
Microsoft.UI.Text\Microsoft\UI\Text\RichEditTextRange
Microsoft.Web.WebView2.Core\Microsoft\Web\WebView2\Core\CoreWebView2
Microsoft.Web.WebView2.Core\Microsoft\Web\WebView2\Core\CoreWebView2Environment
Microsoft.Web.WebView2.Core\Microsoft\Web\WebView2\Core\CoreWebView2WebResourceRequest
Microsoft.Web.WebView2.Core\Microsoft\Web\WebView2\Core\CoreWebView2WebResourceResponse
Microsoft.Web.WebView2.Core\Microsoft\Web\WebView2\Core\CoreWebView2WebResourceResponseView
Microsoft.Web.WebView2.Core\Microsoft\Web\WebView2\Core\ICoreWebView2
Microsoft.Web.WebView2.Core\Microsoft\Web\WebView2\Core\ICoreWebView2Environment
Microsoft.Web.WebView2.Core\Microsoft\Web\WebView2\Core\ICoreWebView2WebResourceRequest
Microsoft.Web.WebView2.Core\Microsoft\Web\WebView2\Core\ICoreWebView2WebResourceResponse
Microsoft.Web.WebView2.Core\Microsoft\Web\WebView2\Core\ICoreWebView2WebResourceResponseView

[jonwis]

Check out https://docs.microsoft.com/en-us/windows/win32/api/shcore/nf-shcore-createrandomaccessstreamonfile and https://docs.microsoft.com/en-us/windows/win32/api/shcore/nf-shcore-createrandomaccessstreamoverstream which should get you an IRandomAccessStream over either a file path or an IStream of your choosing.

[ghost]

• Fix inability to accept long paths (\?)

good luck with enforcing security at the subsystem level when apps have direct access to NT UNC(read unsafe) paths.

[riverar]

Access checks can still be performed as usual. It's also irrelevant for full trust desktop apps, which are all Project Reunion apps at the moment.