Add desktop implementations of file_selector

[stuartmorgan]

Adds desktop implementations of the file_selector plugin. These will temporarily be hosted here, rather than flutter/plugins, because we don't have a testing strategy for native UI in desktop plugins, so don't meet the testing requirements for flutter/plugins (whereas we already have file_chooser here without tests, so this leaves us no worse off).

While this looks like a lot of new code, there's not actually much that's new:

• The plugin code is almost a direct copy of file_chooser. Changes:
  • The classes are renamed
  • The details of the method channel API have changed slightly (different method and dictionary key names, some methods return a single item rather than everything returning a list, type filters have a different serialization structure)
  • macOS now supports UTIs in addition to extensions
  • Linux now supports mime types in addition to extensions
  • Fixed an overrelease issue in the Linux implementation of filter construction
• The example app is just a copy of the example that was landed in the app-facing file_selector plugin, with very minor changes:
  • Changed to use file_selector_platform_interface, rather than the app-facing package, since the interface should be more stable
  • Generated the Windows, macOS, and Linux projects with flutter create .

This initial implementation deliberately uses file_chooser code as much as possible to make for a simple transition to file_selector. Longer term we should consider:

• Rewriting the macOS plugin in Swift
• Rewriting the Windows plugin in Dart with the win32 package.

[stuartmorgan]

Unfortunately despite the native plugin code being largely identical to file_chooser, the class renaming was apparently enough delta that git isn't picking it up as a copy+modification.

[stuartmorgan]

(Once this lands I'll publish these plugins, under my account for now; once that's done and file_selector itself is published, I'll remove file_chooser.)

[stuartmorgan]

Since file_selector is published, and the changes here are minor, I'm going to go ahead and land this so I can publish the implementations so that people can start transitioning to file_selector. @cbracken if you want to do a post-landing review, I can address anything in a follow-up; if not, there will be another review point when it's time to move this to flutter/plugins.