Add performance tests

It would be good to have some kind of performance tests to get a better understanding of how well the extension performs, and to keep track of it to not overlook accidental performance decreases when adding new things.

The performance tests as a start could consist of:

• One autocompletion inside script (means svelte2tsx and the TS service are tested)
• One diagnostic run on a file that needs no preprocessing
• One diagnostic run on a file with TypeScript that needs script preprocessing