Update imports to use explicit file extensions

[mattrunyon]

ES Module imports need explicit extensions per the spec. Without file extensions, this is usually resolved by Webpack, but technically import foo from './foo' is looking for the file foo and not foo.js.

TypeScript will not add the extension for you in TS files (microsoft/TypeScript#16577), but it is valid to import foo from './foo.js' in TS even if foo is a TS file.

We are using a babel plugin to add the extensions currently (#188). This is fine, but it would be nice to not rely on the small package with relatively few downloads.

Adding the file extension to all imports will require some update to eslint configs to fix the import resolution rules