feat: Elm language support (#667)

Also added Node.js setup to GitHub Actions workflow

---------

Co-authored-by: Jordan Munch O'Hare <git@jordan.munchohare.com>

test/solidlsp/elm/test_elm_basic.py

@@ -0,0 +1,54 @@
+import os
+
+import pytest
+
+from solidlsp import SolidLanguageServer
+from solidlsp.ls_config import Language
+from solidlsp.ls_utils import SymbolUtils
+
+
+@pytest.mark.elm
+class TestElmLanguageServer:
+    @pytest.mark.parametrize("language_server", [Language.ELM], indirect=True)
+    def test_find_symbol(self, language_server: SolidLanguageServer) -> None:
+        symbols = language_server.request_full_symbol_tree()
+        assert SymbolUtils.symbol_tree_contains_name(symbols, "greet"), "greet function not found in symbol tree"
+        assert SymbolUtils.symbol_tree_contains_name(symbols, "calculateSum"), "calculateSum function not found in symbol tree"
+        assert SymbolUtils.symbol_tree_contains_name(symbols, "formatMessage"), "formatMessage function not found in symbol tree"
+        assert SymbolUtils.symbol_tree_contains_name(symbols, "addNumbers"), "addNumbers function not found in symbol tree"
+
+    @pytest.mark.parametrize("language_server", [Language.ELM], indirect=True)
+    def test_find_references_within_file(self, language_server: SolidLanguageServer) -> None:
+        file_path = os.path.join("Main.elm")
+        symbols = language_server.request_document_symbols(file_path)
+        greet_symbol = None
+        for sym in symbols[0]:
+            if sym.get("name") == "greet":
+                greet_symbol = sym
+                break
+        assert greet_symbol is not None, "Could not find 'greet' symbol in Main.elm"
+        sel_start = greet_symbol["selectionRange"]["start"]
+        refs = language_server.request_references(file_path, sel_start["line"], sel_start["character"])
+        assert any("Main.elm" in ref.get("relativePath", "") for ref in refs), "Main.elm should reference greet function"
+
+    @pytest.mark.parametrize("language_server", [Language.ELM], indirect=True)
+    def test_find_references_across_files(self, language_server: SolidLanguageServer) -> None:
+        # Test formatMessage function which is defined in Utils.elm and used in Main.elm
+        utils_path = os.path.join("Utils.elm")
+        symbols = language_server.request_document_symbols(utils_path)
+        formatMessage_symbol = None
+        for sym in symbols[0]:
+            if sym.get("name") == "formatMessage":
+                formatMessage_symbol = sym
+                break
+        assert formatMessage_symbol is not None, "Could not find 'formatMessage' symbol in Utils.elm"
+
+        # Get references from the definition in Utils.elm
+        sel_start = formatMessage_symbol["selectionRange"]["start"]
+        refs = language_server.request_references(utils_path, sel_start["line"], sel_start["character"])
+
+        # Verify that we found references
+        assert refs, "Expected to find references for formatMessage"
+
+        # Verify that at least one reference is in Main.elm (where formatMessage is used)
+        assert any("Main.elm" in ref.get("relativePath", "") for ref in refs), "Expected to find usage of formatMessage in Main.elm"