getOuterTokenAlias(): string {
    const index = nullthrows(
      this.parent,
      'Parent node must be defined',
    ).children.indexOf(this);
    invariant(
      index != -1,
      "Could not find current fbt node among the parent node's children",
    );
    return convertIndexInSiblingsArrayToOuterTokenAlias(index);
  }
We define an FbtImplicitParamNode's outer token alias to be
string concatenation of '=m' + the FbtImplicitParamNode's index in its siblings array.

@example For string <fbt> hello <a>world</a></fbt>,
         the outer token alias of <a>world</a> will be '=m1'.