Turn on `noFallthroughCasesInSwitch` setting

https://www.typescriptlang.org/tsconfig#noFallthroughCasesInSwitch

This caused a bug earlier: [`5fb6852` (#132)](https://github.com/sourcegraph/scip-python/pull/132/commits/5fb68522583b0a41ef6f437d76cf5678489eccf0)