fix: Update new project dialog when Dart SDK changes

[tomcra]

This PR provides fixes for

• Create Project Dialog does not render correctly if it is open before SDK location is set #213
• Template options do not update when changing the DART SDK in the Create Project dialog #214

The changes predonmently change the way that the "New Project" dialog UI is updated when the Dart SDK combobox is modified.

The current issues are documented in the linked issues.

Manual Testing included

• Initially setting the SDK directory when it had never been set before
• Setting the SDK directory to a different valid directory version
• Setting the SDK directory to a different invalid directory
• Setting the SDK directory to a different valid directory from and invalid directory
• Creating a project under each of the above scenarios and ensuring the project file structure is created correctly

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[tomcra]

@helin24 or @jwren would you be able to review this pull request? Looking at other recently submitted pull requests it looks like the dart-analysis-server-tests for macos-latest fail regularly.