fix: explicitly add types to exports

[LukasHeimann]

The types are exported and should be able to be used by library consumers. However, when these consumers use recommended TypeScript moduleResolution setting "node16" (https://www.typescriptlang.org/tsconfig#moduleResolution), that will force the resolution to follow what is declared in the exports section of package.json -- where an explicit export of the types is currently missing.

Note that this is different from the types field at the top-level of package.json, which is a legacy mechanism for the old module resolution (this comment explains it pretty well for a different project, which dropped support for the legacy approach: typescript-eslint/typescript-eslint#7186 (comment)).

As wdi5 is released with dual packaging (commonjs + esm), and both version should support typescript, keeping the top-level types field around makes sense. This is also what the wdio-reporter plugin does: https://github.com/webdriverio/webdriverio/blob/d3e494d089d0545cf14ca88043d9599eaf98fa53/packages/wdio-reporter/package.json#L35

They even go a step further and have subpath exports combined with the conditional exports that wdi5 has at the moment (see https://nodejs.org/api/packages.html#subpath-exports and https://nodejs.org/api/packages.html#conditional-exports). Given that in my project, the conditional export only approach works fine, I have opted to not add the subpath variant as well in this PR.

[LukasHeimann]

Tested against a local project of mine (npm link), where this resolved the issue of importing the types in my code.