Fix TypeScript ESM compatibility

[benquarmby]

This PR adds missing .js extensions to imports so that ESM + TypeScript consumers don't hit these errors:

../node_modules/@wry/caches/lib/strong.d.ts(1,34): error TS2835: Relative import paths need explicit file extensions in ECMAScript imports when '--moduleResolution' is 'node16' or 'nodenext'. Did you mean './common.js'?
../node_modules/@wry/caches/lib/weak.d.ts(1,34): error TS2835: Relative import paths need explicit file extensions in ECMAScript imports when '--moduleResolution' is 'node16' or 'nodenext'. Did you mean './common.js'?

Fixes #646