feat: add WindowProxy to file drop handler closure

[chippers]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Documentation
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes. see below
• No

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
Needed so that we can know which window the file drop event occurred in when tauri passes the FileDropHandler to wry. Unless I misunderstand how it's supposed to know the window.

linux only for right now. i took a look at the osx implementation to make sure the proxy works there too but i immediately closed that file lol. didn't check windows either.

ready for not-wip when those 2 are supported

[chippers]

From what I can tell, windows and osx should be supported now. I don't have a machine to test it on personally right now though. Everywhere WindowRpcHandler is used, so is WindowFileDropHandler.