Add Elm highlights.scm

[jcs090218]

This highlights.scm works but it didn't apply the highlighting with tree-sitter-hl-mode. 😕 (No error, no warning)

Parser seems to work correctly:

-- This is a single line comment.

{-
This is a multi-line comment.
It is {- nestable. -}
-}

-- Here we define a value named `greeting`. The type is inferred as a `String`.
greeting =
    "Hello World!"

file:
  line_comment:
  block_comment:
  line_comment:
  value_declaration:
    function_declaration_left:
      lower_case_identifier:
    eq:
    string_constant_expr:
      open_quote:
      regular_string_part:
      close_quote: