Added `as` and `move` keywords to Vim syntax file

`as` (already for a long time) and `move` (which was only added recently, AFAIK) are not marked as keywords in Vim syntax file, so they are not highlighted as keywords in Rust sources. This PR fixes this.
`as` is already a `rustOperator` (see line 17), so marking it as a keyword as well is unnecessary. `as` is already highlighted in Vim today.