JSSpecCompiler: Start converting crashes to error messages

I got fed up with looking at error messages that tell me "VERIFICATION
FAILED: !is_error()". So this commit introduces DiagnosticEngine class
whose purpose is to accumulate and print more user-friendly errors.

Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/DiagnosticEngine.cpp

@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2024, Dan Klishch <danilklishch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "DiagnosticEngine.h"
+
+namespace JSSpecCompiler {
+
+bool DiagnosticEngine::has_fatal_errors() const
+{
+    return m_has_fatal_errors;
+}
+
+void DiagnosticEngine::print_diagnostics()
+{
+    auto use_color = isatty(STDERR_FILENO) ? UseColor::Yes : UseColor::No;
+
+    StringBuilder builder;
+    for (auto const& diagnostic : m_diagnostics)
+        diagnostic.format_into(builder, use_color);
+
+    out(stderr, "{}", builder.string_view());
+}
+
+void DiagnosticEngine::Diagnostic::format_into(StringBuilder& builder, UseColor use_color) const
+{
+    if (!location.filename.is_empty())
+        builder.appendff("{}:{}:{}: ", location.filename, location.line + 1, location.column + 1);
+
+    static constexpr Array<StringView, 4> colored_diagnostic_levels = { {
+        "\e[1mnote\e[0m"sv,
+        "\e[1;33mwarning\e[0m"sv,
+        "\e[1;31merror\e[0m"sv,
+        "\e[1;31mfatal error\e[0m"sv,
+    } };
+    static constexpr Array<StringView, 4> diagnostic_levels = { {
+        "note"sv,
+        "warning"sv,
+        "error"sv,
+        "fatal error"sv,
+    } };
+
+    auto diagnostic_level_text = (use_color == UseColor::Yes ? colored_diagnostic_levels : diagnostic_levels);
+    builder.appendff("{}: ", diagnostic_level_text[to_underlying(level)]);
+
+    if (auto logical_location = location.logical_location) {
+        if (!logical_location->section.is_empty()) {
+            builder.appendff("in {}", logical_location->section);
+            if (!logical_location->step.is_empty())
+                builder.appendff(" step {}", logical_location->step);
+            builder.appendff(": ");
+        }
+    }
+
+    builder.append(message);
+    builder.append('\n');
+
+    for (auto const& note : notes)
+        note.format_into(builder, use_color);
+}
+
+void DiagnosticEngine::add_diagnostic(Diagnostic&& diagnostic)
+{
+    if (diagnostic.level == DiagnosticLevel::FatalError)
+        m_has_fatal_errors = true;
+    if (diagnostic.level != DiagnosticLevel::Note)
+        m_diagnostics.append(move(diagnostic));
+    else
+        m_diagnostics.last().notes.append(move(diagnostic));
+}
+
+}