Change default Context type to unknown

[novemberborn]

Previously, the Context was typed as an empty object. This suddenly started giving problems when typing t.try() (#1947 (comment)). Setting it to unknown seems reasonable.

This may break some code, but I'm not sure how you'd use context without typing it, and IIRC there was a non-major TypeScript change that led to similar breakage regarding the {} type. It seems like the kind of breakage that TypeScript users are used to, so we shouldn't need a major bump to release this. @sindresorhus?

[sindresorhus]

This is the default now in TS too: https://devblogs.microsoft.com/typescript/announcing-typescript-3-5/

Generic type parameters are implicitly constrained to unknown

[vintprox]

Used along with TypeScript, context of unknown type triggers errors when I try to add property to it. Shouldn't it be of any type?

[novemberborn]

You can type context with some type casts, see https://github.com/avajs/ava/blob/master/docs/recipes/typescript.md#typing-tcontext. I'm working on a new way of building test() functions where the context can be inferred, keep an eye on #2435 for that.