Expression `xtokenize`'s `Mode` property is counterintuitive single letter dropdown

[igarashitm]

Please describe the task that needs to be done

In the schema it's just an enum of [i, w, u, t] and uniforms is just showing it. We should be able to make this more human readable by:

1. Add a tooltip help for Mode property, which shows the property description. It explains which letter is for what
2. Enhance enum dropdown in the uniforms to show a description under the letter, just like we do for languages

[igarashitm]

Now the description shown by clicking help icon explains it