fix(interpolation/v-for): allow optional index parameter for numeric iteration

[thebanjomatic]

The v-for directive when iterating over a number provides both the 1-based numeric value, and the 0-based index to the iteration function. Vetur currently shows this as an error when you are using the experimental.templateInterpolationService option and attempt to use the index.

Existing behavior:

No overload matches this call.
  The last overload gave the following error.
    Argument of type 'number' is not assignable to parameter of type 'object'.Vetur(2769)

This change adds the missing second parameter to the iteration helper for the num: number overload, and existing usages with a single parameter should continue to work.

[thebanjomatic]

@yoyo930021 sorry to ping you, but I wasn't sure if you've seen this PR yet.

[yoyo930021]

LGTM