syntax: introduce optional chaining operators

Introduce new operators `?.`, `?.[…]` and `?.(…)` to simplify looking up
deeply nested property chain in a secure manner.

The `?.` operator behaves like the `.` property access operator but yields
`null` if the left hand side is `null` or not an object.

Like `?.`, the `?.[…]` operator behaves like the `[…]` computed property
access but yields `null` if the left hand side is `null` or neither an
object or array.

Finally the `?.(…)` operator behaves like the function call operator `(…)`
but yields `null` if the left hand side is `null` or not a callable
function.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

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