Add more keywords customization support

[AndyWing]

add customization for:

• open
• public private protected internal inner
• data
• sealed
• abstract
• infix
• companion
• override
• this
• super
• null
• suspend
• true/false

known bug for some keywords cannot highlight, tracking cr in Jetbrains-KT-41574

[lppedd]

Hi! Sorry for the delay, and thanks 😄
Will give this a look tonight to see if this is worth adding to the plugin.

Edit: seen the issue. I think we need to consider a subset of the ones you proposed.