Reads an IF statement

```ebnf
if ::= T_IF '(' expr ')' ':' ...
```
function() {
    const result = this.node("if");
    let body = null;
    let alternate = null;
    let shortForm = false;
    let test = null;
    test = this.next().read_if_expr();

    if (this.token === ":") {
      shortForm = true;
      this.next();
      body = this.node("block");
      const items = [];
      while (this.token !== this.EOF && this.token !== this.tok.T_ENDIF) {
        if (this.token === this.tok.T_ELSEIF) {
          alternate = this.read_elseif_short();
          break;
        } else if (this.token === this.tok.T_ELSE) {
          alternate = this.read_else_short();
          break;
        }
        items.push(this.read_inner_statement());
      }
      body = body(null, items);
      this.expect(this.tok.T_ENDIF) && this.next();
      this.expectEndOfStatement();
    } else {
      body = this.read_statement();
      if (this.token === this.tok.T_ELSEIF) {
        alternate = this.read_if();
      } else if (this.token === this.tok.T_ELSE) {
        alternate = this.next().read_statement();
      }
    }
    return result(test, body, alternate, shortForm);
  }