build(aio): create minLength content rule (#22759)

This rule can be used to ensure that properties contain a minimum
number of characters.

PR Close #22759

aio/tools/transforms/angular-api-package/content-rules/minLength.spec.js

@@ -0,0 +1,19 @@
+const createMinLengthRule = require('./minLength');
+
+describe('createMinLength rule', () => {
+
+  const defaultRule = createMinLengthRule();
+  const atLeast5CharsRule = createMinLengthRule(5);
+
+  it('should return `undefined` if the length of the property value is long enough', () => {
+    expect(defaultRule({}, 'description', 'abc')).toBeUndefined();
+    expect(atLeast5CharsRule({}, 'description', 'abcde')).toBeUndefined();
+  });
+
+  it('should return an error message if length of the property value is not long enough', () => {
+    expect(defaultRule({}, 'description', 'a'))
+      .toEqual('Invalid "description" property: "a". It must have at least 2 characters.');
+    expect(atLeast5CharsRule({}, 'description', 'abcd'))
+      .toEqual('Invalid "description" property: "abcd". It must have at least 5 characters.');
+  });
+});