Set up strict boolean checks in linter

haha yeah good point.  I think [this](https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/strict-boolean-expressions.md) would do the trick?

_Originally posted by @pokey in https://github.com/pokey/cursorless-vscode/pull/56#discussion_r665663654_