add auto-highlighting of selected/updated code

src/application/State/Code/Position/CodePosition.ts

@@ -0,0 +1,24 @@
+import { ICodePosition } from './ICodePosition';
+export class CodePosition implements ICodePosition {
+
+    public get totalLines(): number {
+        return this.endLine - this.startLine;
+    }
+
+    constructor(
+        public readonly startLine: number,
+        public readonly endLine: number) {
+        if (startLine < 0) {
+            throw new Error('Code cannot start in a negative line');
+        }
+        if (endLine < 0) {
+            throw new Error('Code cannot end in a negative line');
+        }
+        if (endLine === startLine) {
+            throw new Error('Empty code');
+        }
+        if (endLine < startLine) {
+            throw new Error('End line cannot be less than start line');
+        }
+    }
+}