Null Coalescing expressions should handle `<uninitialized>`

Proposal

Enhance the null coalescing operator to also check for <uninitialized>. Either invalid or <uninitialized> will result in the alternate being selected. Example:

thing = notDefined
result = thing ?? "default"

Will now transpile to:

thing = notDefined
result = thing
if type(thing) = "<uninitialized>" or thing = invalid
    result = "default"
end if

Justification:

Developer intent for null coalescing is to pick an alternate value if the consequent value is not defined. invalid and <uninitialized> both represent this concept of "not defined", and as such, we can produce safer code while maintaining the intent behind null coalescing.

Example:

thing = notDefined
result = thing ?? "default"

In the current releases of BrighterScript, this throws an exception when comparing thing (which is <uninitialized>) to invalid

Questions:

• should this be considered a breaking change or a bugfix? (i.e. should this only be added to v1, or can we also add to v0)

Read docs/null-coalescing-operator.md and reproduce the issue's ?? example with an uninitialized value. Trace the null-coalescing transpilation entry point, then verify that both invalid and <uninitialized> select the alternate without an exception; the issue also leaves the v0 versus v1 decision open.