Correct highlight of replaced variables/splats with braces

[msftrncs]

• Prevent adjustment of userCompletionText in FindUserCompletionTextPosition when the match contains { in CompletionText[1] if the userCompletionText is not longer than 1 char
• Preserve the splat sigil at start of completions of type Variable.

Adjusting userCompletionText blindly led to ArgumentOutOfRangeException on incorrectly braced splats on input @?.
Loss of the sigil of splats led to the splat sigil always being marked as replaced, and in situations where the PowerShell completion logic returned incorrect completions of splats with braces, led to a ArgumentOutOfRangeException.

Fix #983.

[msftrncs]

I think I figured out what the check for a { is doing … I'll probably implement a different correction.

The detection for a { appears to be to handle the situation where @en is the input, and then when @{env:blah(blah)} is the completion, the selection becomes 'v:blah(blah)}`, not selecting the first brace.

[msftrncs]

I've amended the commit for this PR. Its very possible that either of the two changes would stop the ArgumentOutOfRangeException from occuring. The correction for maintaining the sigil of the returned completions is probably the item that solves the problem, but the other change is a safety net. (See comments below)

[daxian-dbw]

LGTM

[daxian-dbw]

The correction for maintaining the sigil of the returned completions is probably the item that solves the problem, but the other change is a safety net.

The correction for maintaining the sigil of the returned userCompletionText is the item that solves the ArgumentOutOfRangeException, but the other change is needed to have the correct selection for ${$} when typing echo $Ctrl+Space.

[msftrncs]

@daxian-dbw, you are right, I was so focused on the @ sigil, I overlooked the importance the other change (to FindUserCompletionTextPosition) had on the $ selection when the { was present in the completion.