Fix escape rgx_map for Otp 28

[bopm]

On Elixir 1.18.4 with Erlang/OTP 28 project fails to compile:

== Compilation error in file lib/earmark_parser/line_scanner.ex ==
** (ArgumentError) cannot inject attribute @rgx_map into function/macro because cannot escape #Reference<0.983345734.4272029698.15857>. The supported values are: lists, tuples, maps, atoms, numbers, bitstrings, PIDs and remote functions in the format &Mod.fun/arity
    (elixir 1.18.4) lib/kernel.ex:3777: Kernel.do_at_escape/2
    (elixir 1.18.4) expanding macro: Kernel.@/1
    lib/earmark_parser/line_scanner.ex:334: Earmark.Parser.LineScanner.regex_run/3
    (elixir 1.18.4) expanding macro: Kernel.|>/2
    lib/earmark_parser/line_scanner.ex:335: Earmark.Parser.LineScanner.regex_run/3
    (elixir 1.18.4) expanding macro: Kernel.|>/2
    lib/earmark_parser/line_scanner.ex:336: Earmark.Parser.LineScanner.regex_run/3
could not compile dependency :earmark, "mix compile" failed. Errors may have been logged above. You can recompile this dependency with "mix deps.compile earmark --force", update it with "mix deps.update earmark" or clean it with "mix deps.clean earmark"

This issue is similar to this and this.
This PR fixes that.

[walkr]

I propose defp rgx_map be replaced by a private macro. It's faster since it avoids a function call (which might make a difference in tight loops):

defmacrop rgx_macro do
  quote do
   # ...
  end 
end

[walkr]

Never mind, I've just run some benchmarks, and it appears that in this scenario the difference between the macro and the function is negligible.

[adamu]

I believe this is a subset of the changes from #512 that was opened in March.