Intellisense: offer new completions after selecting a completion with a triggering character

[isc-tleavitt]

For example:
select * from (offers schemas)
pick a schema, get:
select * from MySchema.

To get table completions offered, I need to backspace the . and retype it, THEN it'll offer table suggestions. I shouldn't need to do that.

(from a bit of digging, this might be upstream all the way in VSCode.)

[gjsjohnmurray]

@isc-tleavitt is this a consequence of the way your recent PR for schemas adds the . at the end of what it inserts?

[isc-tleavitt]

@gjsjohnmurray I suppose this wouldn't be an issue if the user needed to type the . in the first place. (But I don't think the user should need to.)

[gjsjohnmurray]

Maybe microsoft/vscode#82236 is relevant.

Another issue linked from there mentions how some extensions use CompletionItem.command to trigger a second set of suggestions. I'm guessing they run editor.action.triggerSuggest but I haven't verified this.