compiler, runtime: Support Formatted Error Messages

Make it possible to output formatted error messages using a
flexible API that includes an ability to specify styles and
formatters.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Tests/p4rseTests/StyleTests/Style.swift

@@ -0,0 +1,57 @@
+// p4rse, Copyright 2026, Will Hawkins
+//
+// This file is part of p4rse.
+//
+// This file is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+import Common
+import Foundation
+import Macros
+import P4Runtime
+import P4Lang
+import SwiftTreeSitter
+import Testing
+import TreeSitter
+import TreeSitterP4
+
+@testable import P4Compiler
+
+@Test func test_style_add_format() async throws {
+    let red = Style(StyleColor.Red)
+    let bold_red = red.update(addFormat: StyleFormat.Bold)
+
+    #expect(bold_red == Style(StyleColor.Red, [StyleFormat.Bold]))
+}
+
+@Test func test_style_add_format2() async throws {
+    let bold_red = Style(StyleColor.Red, [StyleFormat.Bold])
+    let bold_underline_red = bold_red.update(addFormat: StyleFormat.Underline)
+
+    #expect(bold_underline_red == Style(StyleColor.Red, [StyleFormat.Bold, StyleFormat.Underline]))
+}
+
+
+@Test func test_style_remove_format() async throws {
+    let bold_red = Style(StyleColor.Red, [StyleFormat.Bold])
+    let red = bold_red.update(removeFormat: StyleFormat.Bold)
+
+    #expect(red == Style(StyleColor.Red))
+}
+
+@Test func test_style_remove_format2() async throws {
+    let bold_underline_red = Style(StyleColor.Red, [StyleFormat.Bold, StyleFormat.Underline])
+    let underline_red = bold_underline_red.update(removeFormat: StyleFormat.Bold)
+
+    #expect(underline_red == Style(StyleColor.Red, [StyleFormat.Underline]))
+}