Use .ts files

[stazz]

Currently the code uses .mts files to signify that the output is ESM code, and to make IDE automatically use .mjs extension in imports/exports.

However, since the eslint-plugin-path-import-extension has been released, it probably will be better to just stick with .ts files, and let ESLint plugin handle the extension for output files. The package.json specification of "type": "module" should ensure that Node runs the code in ESM mode.