Support for typed template slots

[johnsoncodehk]

Since 0.34.0, typed template slots is not supported due to #1108, we need to wait for vuejs/core#2693 implemented to have a new way to support typed template slots that do not have performance issue.

[oliverzy]

Could we make this feature as an option that default as false and add a warning if user want to enable it? Because I see this upstream issue has been opened almost one and half years, typed slot is an very important feature that volar has

[TotomInc]

I agree with @oliverzy, the best of both worlds would be to make it an option that is turned off by default.