feat(apollo-client): Adds type vars to subscribeToMore for variables

[timhwang21]

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

Addresses #4246.

I was having some issues installing all needed dependencies, but this is just a type change and TSC doesn't show any errors for me.

[apollo-cla]

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[timhwang21]

I wasn't able to run the test locally, and it seems like the failure is just due to me not setting the tests up locally. The only thing the test does is add some dummy variables to the query and the subscription. Would really appreciate it if a maintainer could take a look!

[benjamn]

Two notes:

• The tests are still failing after merging master into this branch, so you might want to take another look?
• I merged feat(apollo-client): Adds type vars to subscribeToMore for variables #4608 which contains very similar changes to subscribeToMore, though I would like to merge this PR as well, to include the tests you wrote. Rebase when you have a chance, please?

[timhwang21]

Sounds good -- I just rebased and dropped duplicate changes (though I kept a rename of TVariables to TSubscriptionVariables as I think it's more accurate). Will try to solve the test failure next, but was hoping you can take a look as well, since the issue is definitely with my test and me not configuring it properly.

The only thing the test is testing anyways is a situation that should typecheck...