Optional chaining

[mariusGundersen]

This adds support for the three types of optional chaining:

a?.b // returns b unless a is nullish
a?.() // calls the function a unless a is nullish
a?.[0] // returns the first item in the array a unless a is nullish

[mariusGundersen]

This is wished for here: #1302 (comment)

[gfwilliams]

I just noticed these spaces got replaced with tabs. Please can you try and tweak these (and change your editor so tabs don't get added?). It really screws up the layout if someone views the code and doesn't have 8 space tabs set

[gfwilliams]

I'm afraid this breaks stuff. undefined[1] should cause an exception (like 1143) but JSP_RESTORE_EXECUTE removes the error flag, so it just quietly completes. I can imagine there might be other code you could come up with (not just errors, but return/break/etc) too

[gfwilliams]

Sorry it's taken so long to look into this! But I am a bit concerned about the use of JSP_SAVE_EXECUTE/RESTORE - they need to be used with real care so you don't accidentally remove execution state. I think it might be possible to make it work without them though, as all you care about is executing or not (rather than any of the other state)