Improve export of sub-folders

[Nick-Lucas]

1. Currently the folders are under /src/lib/validators which isn't ergonomic
2. webpack fails to load sourcemaps in these folders with an error like:

WARNING in ../../../node_modules/@gapstack/light-type/src/lib/validators/strings.js
Module Warning (from ../../../node_modules/source-map-loader/dist/cjs.js):
Failed to parse source map from '...../packages/light-type/src/lib/validators/strings.ts' file: Error: ENOENT: no such file or directory, open '....../packages/light-type/src/lib/validators/strings.ts'