Reads an interface body
```ebnf
interface_body ::= (member_flags? (T_CONST | T_FUNCTION))*
```
function() {
    let result = [];

    while (this.token !== this.EOF && this.token !== "}") {
      if (this.token === this.tok.T_COMMENT) {
        result.push(this.read_comment());
        continue;
      }

      if (this.token === this.tok.T_DOC_COMMENT) {
        result.push(this.read_doc_comment());
        continue;
      }

      // read member flags
      const flags = this.read_member_flags(true);

      // check constant
      if (this.token == this.tok.T_CONST) {
        const constants = this.read_constant_list(flags);
        if (this.expect(";")) {
          this.next();
        }
        result = result.concat(constants);
      } else if (this.token === this.tok.T_FUNCTION) {
        // reads a function
        const method = this.read_function_declaration(2, flags);
        method.parseFlags(flags);
        result.push(method);
        if (this.expect(";")) {
          this.next();
        }
      } else {
        // raise an error
        this.error([this.tok.T_CONST, this.tok.T_FUNCTION]);
        this.next();
      }
    }
    if (this.expect("}")) {
      this.next();
    }
    return result;
  }