[Proposal] Make the Dim keyword optional

For example, as an alternative to:

`Dim a As Integer`

also recognise:

`a As Integer`

Possibly the compiler/parser could use the `As` keyword to recognise that a variable is being declared? This would help to make VB.NET more concise.

No files, tests, or entry points are named. Start by reviewing the compiler/parser handling of Dim and As declarations, then define how `a As Integer` should be recognised and what declaration cases and ambiguities must be covered before the proposal is complete.

Written by the indexing model from the issue text.