Reads a use statement
```ebnf
use_alias ::= (T_AS T_STRING)?
```
@return {String|null}
function() {
    let result = null;
    if (this.token === this.tok.T_AS) {
      if (this.next().expect(this.tok.T_STRING)) {
        const aliasName = this.node("identifier");
        const name = this.text();
        this.next();
        result = aliasName(name);
      }
    }
    return result;
  }