Improve the set of export commands

[yhatt]

from: #87, #203

Following commands are refreshed suggestions based on #463 (comment).

• Export Slide Deck (Quick Export) (markdown.marp.exportQuick)
  • Create the output next to the current file.
    • If the target file was not saved, this command has the same meaning as "Export slide deck as..."
  • The format is specified in markdown.marp.exportType preference.

• Export Slide Deck To Selected Format ... (markdown.marp.exportToSelectedFormat)
  • Execute quick export, but ask for the file format before.
  • It could be based on the implementation by Added the "Quick Export" Command #463.

• Export Slide Deck As ... (markdown.marp.exportAs / markdown.marp.export for backward compatibility)
  • Same as the current command Export Slide Deck....
  • Always open save dialog, and ask where to export.
  • User can choose the format in the save dialog.

#203 suggests adding an extra directive for specifying a file name in the quick export. Honestly we don't want to increase the number of directives because newbies may be overwhelmed by many directives.

Legacy suggestion

• "Export to ***": Quick export commands ("to PDF", "to HTML", "to PPTX"). The file name is the same as current document, and it will be exported next to the document.

[infogulch]

An option that I would like is quick export to a temp file that is cleaned up later. Maybe labeled "Present with HTML" / "Present with PDF" / ...

[yhatt]

The suggestion was updated that is based on #463 (comment). The legacy suggestion is corresponding to markdown.marp.exportToSelectedFormat.