Fix snippet placeholder formatting
Fix snippet placeholder formatting

This change fixes some ambiguity in our snippet placeholders to confirm
with stricter parsing rules in VS Code 1.12.0.  Generally this requires
prefixing an index (`${1:Placeholder Text}`) anywhere that a placeholder
is used so that the syntax isn't ambiguous with a variable name.
Snippet variables are a separate concept now.

Resolves #767.

PowerShell/vscode-powershell