feat: Add Julia language server support (#691)

Implements JuliaLanguageServer with LanguageServer.jl integration.
Includes comprehensive tests and auto-detection of Julia installations.

Checks for LanguageServer.jl on startup and installs via Pkg.add if not found. Only runs once on first use.

test/solidlsp/julia/test_julia_basic.py

@@ -0,0 +1,50 @@
+import pytest
+
+from solidlsp.ls import SolidLanguageServer
+from solidlsp.ls_config import Language
+
+
+@pytest.mark.julia
+class TestJuliaLanguageServer:
+    @pytest.mark.parametrize("language_server", [Language.JULIA], indirect=True)
+    def test_julia_symbols(self, language_server: SolidLanguageServer):
+        """
+        Test if we can find the top-level symbols in the main.jl file.
+        """
+        all_symbols, _ = language_server.request_document_symbols("main.jl")
+        symbol_names = {s["name"] for s in all_symbols}
+        assert "calculate_sum" in symbol_names
+        assert "main" in symbol_names
+
+    @pytest.mark.parametrize("language_server", [Language.JULIA], indirect=True)
+    def test_julia_within_file_references(self, language_server: SolidLanguageServer):
+        """
+        Test finding references to a function within the same file.
+        """
+        # Find references to 'calculate_sum' - the function name starts at line 2, column 9
+        # LSP uses 0-based indexing
+        references = language_server.request_references("main.jl", line=2, column=9)
+
+        # Should find at least the definition and the call site
+        assert len(references) >= 1, f"Expected at least 1 reference, got {len(references)}"
+
+        # Verify at least one reference is in main.jl
+        reference_paths = [ref["relativePath"] for ref in references]
+        assert "main.jl" in reference_paths
+
+    @pytest.mark.parametrize("language_server", [Language.JULIA], indirect=True)
+    def test_julia_cross_file_references(self, language_server: SolidLanguageServer):
+        """
+        Test finding references to a function defined in another file.
+        """
+        # The 'say_hello' function name starts at line 1, column 13 in lib/helper.jl
+        # LSP uses 0-based indexing
+        references = language_server.request_references("lib/helper.jl", line=1, column=13)
+
+        # Should find at least the call site in main.jl
+        assert len(references) >= 1, f"Expected at least 1 reference, got {len(references)}"
+
+        # Verify at least one reference points to the usage
+        reference_paths = [ref["relativePath"] for ref in references]
+        # The reference might be in either file (definition or usage)
+        assert "main.jl" in reference_paths or "lib/helper.jl" in reference_paths