Feat: editable dom element (#39341)

* feat: __testEditable allows editable region tests

It's not entirely isolated, but it makes it possible to select only the
element with id 'editable-only' which is built solely from code inside
the editable region.

* fix(client): missing editableContents -> ''

Previously it was added as the string 'undefined'

* fix: more informative error messages

* fix: DRY, correct and test getLines

utils/get-lines.js

@@ -0,0 +1,16 @@
+const { isEmpty } = require('lodash');
+
+// TODO: secure with tests
+function getLines(contents, range) {
+  if (isEmpty(range)) {
+    return '';
+  }
+  const lines = contents.split('\n');
+  const editableLines =
+    isEmpty(lines) || range[1] <= range[0]
+      ? []
+      : lines.slice(range[0], range[1] - 1);
+  return editableLines.join('\n');
+}
+
+exports.getLines = getLines;