Clarify file sharing flags in FML filesystem APIs on Windows

[jason-simmons]

Use shared mode when requesting read access and exclusive mode for write access

[flutter-dashboard]

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[jason-simmons]

This may be related to the errors that ImpellerC is sometimes getting on LUCI when it opens the build output directory (see flutter/flutter#116761)

[chinmaygarde]

Not sure how to interpret this TBH. Why should the file be allowed to be written or deleted as we access it? The exclusive access bit makes sense though.

[jason-simmons]

Currently the Windows implementation of FilePermission::kRead is specifying only FILE_SHARE_READ. This means that Windows will refuse to open the file/directory if some other process has opened it for write access
(see https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea)

This seems too restrictive. For something like ImpellerC's use of fml::OpenDirectory it is unnecessary to require that level of exclusivity just to get a read-only handle to a directory. I suspect the same applies to other file/directory reading scenarios.

FML is not applying this kind of locking for file access on POSIX platforms.

[chinmaygarde]

LGTM as a speculative fix. Not sure how to test this but lets timebox something?

[jason-simmons]

Added a test to verify that fml::OpenDirectory(FilePermission::kRead) works if the directory has also been opened for writing on Windows