Optional Chaining

When looking for a property value that's deep in a tree-like structure, one often has to check whether intermediate nodes exist:

if user.address AND user.address.street then
  street = user.address.street
end if

Also, many API return either an object or Invalid, and one may want to extract a property from the result only when it is not Invalid:

fooInput = m.top.findNode('fooIinput')

if Invalid <> fooInput then
  fooValue = fooInput.value
end if

The Optional Chaining Operator allows a developer to handle many of those cases without repeating themselves and/or assigning intermediate results in temporary variables:

street = user.address?.street
fooValue = m.top.findNode('fooInput')?.value

My examples above are just tweaks to the one on this repo: https://github.com/tc39/proposal-optional-chaining

Start by reviewing the optional chaining proposal linked in the issue and the existing BrightScript compiler support for language syntax. Confirm how the shown nested-property and Invalid-value examples should behave, then define completion around those examples compiling and producing the expected values.