Fix heex-ts-mode indentation following previews elixir-mode change

Fix heex-ts-mode indentation following previews elixir-mode change

After the previous fix in elixir-ts-mode (0fd259d166c), embedded heex
code are indented like this:

1 defmodule Foo do
2   def foo(assigns) do
3     ~H"""
4 <span>
5   text
6 </span>
7     """
8   end
9 end

The indent rule finds the beginning of the parent heex node, and uses
the indentation of that line as anchor with an offset of 0.  Previously
the parent heex node (fragment) starts at EOL of line 3; after the
previous commit, it now starts at BOL of line 4.  To fix the
indentation, I changed the anchor to the beginning of the elixir
(rather than heex) node at point, which is at EOL at line 3.

* lisp/progmodes/heex-ts-mode.el (heex-ts--indent-rules): Use the elixir
node that contains the heex code as the anchor, instead of the heex root
node.