* added partial load mode feature flag and made extract method to handle partial load mode.

when solution is opened in VS, currently we do not know when exactly solution is fully loaded or not. solution explorer might show all files but design time build might not have finished yet so, any semantic operation such as extract method or rename might do wrong thing since it doesn't know that it is dealing with missing information (solution).

we are trying to address this issue with partial load mode, new API platform is working on. basically, we are planning to block most of semantic operations (except completion/quickinfo/goto definition/signature help and navigate to. we might include FAR here as well) during partial load state.

also, for things like squiggle, LB and etc, we will disable those until solution is fully loaded since we don't actually know whether those data is correct or not during partial load state.

this change add featureflag so that we can incrementally add support for each features while dogfooding behavior across partners. as a first attempt, extract method is choosen for new experience.

other internal options are added so that we can try alternative behaviors and etc.

test is not added yet.

* fix merge break

* update string

* small tweak.

* PR feedbacks

* updated the title

* updated after discussing about PR

* do even cheaper version of getting text.

addscope doesn't work with description with blank line when scope is nested. wait dialog seems supporting only 2 lines.

until, I figure out this with platform team, making description one line.

* following PR feedback