-Unable to rename Stylesheet selector using the context menu option.
-Workaround: Rename it from the Inspector.

Steps to repro:
1. Create a new Project.
2. Window > UI Toolkit > UI Builder.
3. StyleSheets > Click on "+" > Create New USS > "Test".
4. Select "Test" > Inspector > Create New USS Selector > MyTest.
5. MyTest > Right Click > Rename

Actual Result:
Nothing happens.

Expected Result:
Selected element should be in the renamed state.

Reproducible in:
2021.1.0a10, 2020.2.0f1, 2020.1.1f1, 2019.4.0f1 with Package version 1.0.0-preview.9.

Environment:
Occurring on Windows 10 & Mac 10.15.