refactor(core): all API are now promise based

[lucasfernog]

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

• Bugfix
• Feature
• New Binding Issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

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

• Yes. Issue #___
• 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:

The Tauri API is now Promise-based everywhere.

promisified was removed (use invoke exclusively now)
tauri::execute_promise helper was removed (the return value is used to resolve/reject the promise instead)
tauri::execute_promise_sync helper was removed (the command message has a mainThread: bool field that allows your command to run on the main thread instead of a green thread.