Close gaps in the complete set of available choices for Razor completion

[phil-allen-msft]

This issue tracks the "completeness" portion of having Razor completion.\

• Completion right after @bind$$-Parameter does not overwrite existing parameter #9756
• Component attribute completions not shown when completion Invoked with caret on an attribute #9728
• Event handler attribute doesn't add quotes after = #9753
• VS missing using keyword in completion list #9331
• Add using keyword completion provider #9330

External:

• List Members has a BUG and if it isn't a BUG then it's a bad design... #9606
• "Await completion adds async modifier" sets caret position after async keyword #9721
• Razor editor - adding await adds async keyword has erroneous behaviour #9942

Unable to repro:

• Typing "too fast" does not trigger member list in C# block #9724
• When typing "too fast", HTML element completions are omitted #9725
• Wrong completion matches temporarily #9802
• HTML elements are missing from tag close completions #10030