Add word completion for reserved terms

[ckhiu]

I would really love to have a extended word completion in the live editor https://mermaid.live/ to support reserved terms.
Until now only words are suggested that were already typed in.
What I would like to see is that also reserved terms are suggested even if you haven't typed them yet.

So for example if you start a sequenceDiagram from scratch, I would like to see word suggestions for:

• title
• participant
• activate
• deactivate
• note
• ...