[EEx] Allow commenting inside case & cond clauses

[tcitworld]

Environment

• Elixir & Erlang/OTP versions (elixir --version): Erlang/OTP 24, Elixir 1.12.1
• Operating system: Fedora 35

Current behavior

The following EEx template doesn't compile when a comment lies between the case/cond do and the first clause. I would expect blocks starting with <%# to be ignored by the compiler and not interpreted as an expression.

EEx.compile_string("""
<%= case true do %>
<%# comment %>
<% true -> %>
  false
<% false -> %>
  true
<% end %>
""")

** (SyntaxError) nofile:4: unexpected operator ->. If you want to define multiple clauses, the first expression must use ->. Syntax error before: '->'

If the comment is placed in the first clause (or anytime later), it works properly.

EEx.compile_string("""
<%= case true do %>
<% true -> %>
  <%# comment %>
  false
<% false -> %>
  true
<% end %>
""")

Expected behavior

I would expect a similar behavior than:

case true do
  # comment
  true -> true
  false -> false
end

It can make sense to put a comment to explain what this clause means before the clause itself.

Possibly related: #10256