Refactor terminal and remove duplicate code.

lib/terminal/enums.dart

@@ -0,0 +1,8 @@
+/// Source category of a terminal entry
+enum TerminalSource { network, js, system }
+
+/// Severity level of a terminal entry
+enum TerminalLevel { debug, info, warn, error }
+
+/// Phase of a network log lifecycle
+enum NetworkPhase { started, progress, completed, failed }