Make NullableContextOptions optional

[kirides]

I would like to have the NullableContextOptions optional, akin to "Optimized"-builds. E.g. a checkbox for "NullableContextOptions"

[kirides]

If someone were to implement this

1. should the parameter be passed using ExecutionHost.ExecuteAsync(..., nullableContextOptions)
2. or should a new _executionHost be created?
3. or do we set a property on the _executionHost every time we call NullableContextOptions_set(value) (property set)

[aelij]

Since it can change every execution, it should be the first option (like optimizationLevel).
But this is not enough. The design-time build also needs to be configured (see RoslynHost and UpdateDocument).