why

Stagehand doesn't have TS strict mode on, which causes confusing things to happen for users of the library (e.g. receiving undefined from a function whose return type is string).

what changed

• "strict": true added to tsconfig.json
• typing + handling throughout Stagehand to pass TS strict mode checks

test plan

Build successfully + validate against existing evaluations to ensure no behavior changes

This is my first pass at it, there's still a bunch of errors left + some errors I don't have enough context to definitively fix (cc. @miguelg719)

I've left comments throughout as comments prefixed // NOTE: , and sectioned out groups of changes into commits so that each commit should be easy to understand for a reviewer