Extend snippets to include optionals

[njames93]

Would it be possible to extend the snippet syntax to support optional snippets.
I'm thinking a syntax for this could be of the form be ${ int ? any } or ${ int * any }.
As an example this is probably how an editor should treat these syntaxes:

• The ? case would be for when its unlikely to be needed and the user should opt in.
  • Tabbing past it should remove the snippet.
  • Hitting return (or starting to type if there is a nested placeholder) should keep it.
• The * case would be for when its expected to be needed and the user should opt out.
  • Tabbing past it should keep the snippet.
  • Hitting delete should remove the snippet.

These would be able to nest items inside them (like how placeholders work) enabling complex optional blocks that can be completely removed with one hit of a button, like in the examples below.
If this is a little too complex it may be better to just have the single ? syntax, the * syntax probably isn't as useful.

Some use cases of this would be:

• Defaulted arguments
  • Say a function has the signature greet(Greeting, Audience="World"), a snippet could look like
    greet(${0:Greeting}${1?, ${2:"World"}})
    As can be seen here, the comma is part of the optional block, So the user doesn't want to specify an explicit argument, the comma will be removed as well, so no malformed code like greet("Bonjour", ).
• Declaring a variable with a type that's not usually necessary, but an initializer which usually is important.
  • let ${0:Name} ${1? : ${2:Type}}${3* = {$4:Initializer}; Again The : and = are in the optional blocks so they would be removed along with Type and Initializer if the user didn't want them.
• A snippet for an if that includes the else.
  • if (${0:Condition}) then ${1:Statement} ${2*else ${3:Statement} }endif
• Modifier on some language construct
  • if ${0?constexpr }(${1:Condition}){${2:Statements}}