Added async, await and yield C# keywords support

[pedrolamas]

I added these originally in eace8e0, but it got removed with 7c60184, so I'm adding them back now!

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[pedrolamas]

I signed it!

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CLAs look good, thanks!

[amroamroamro]

@pedrolamas
FYI, src/prettify.js is auto-generated from the sources in js-modules/*.js,
so your changes will be undone when the makefile is rerun.

I suggest you make modifications to files in js-modules/ in the future.

[pedrolamas]

I did these changes on js-modules/prettify.js, isn't that where they are supposed to go in?

[amroamroamro]

It is, I was talking about your old commit :)

[pedrolamas]

Ah, thanks! Indeed I can say I learned my lesson!! 😄

[amroamroamro]

I fixed those mishaps in this last update #467
(there was your PR #418 and also PR #384 which were mistakenly undone).
This is now safe to close, thank you.