Do not hijack core:save-as

[winstliu]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

Instead of hijacking core:save-as and making it do what we want, instead use the public pane.saveItemAs method combined with item.getSaveDialogOptions to set the filepath we want. Essentially, it delegates the save dialog and error handling to Atom core and only handles saving the file itself once the filepath has been chosen.

Alternate Designs

Keep atom.showSaveDialog[Sync] in Atom core.

Benefits

Less custom logic for how to save a file as HTML.

Possible Drawbacks

I've added a notification to warn when the file can't be saved due to the Markdown still loading. Unfortunately, it will appear after the save dialog finishes. I think this is still better than what we had before, which was no feedback.

Applicable Issues

Refs atom/atom#16245