exclude tests in distribution, test esm and cjs

[heyAyushh]

Problem

• test file was in src folder hence, included in the distribution,
  which increases unnecessary increase in bundle size,
• tests were all in one file, risking E2BIG errors as the file size grew
• difficult to manage and locate specific tests
• running tests in a single file can be inefficient and error-prone

Solution

• moved tests to separate files in the tests folder to prevent file size issues
• seperated test file into multiple test files for better organisation
• added esbuild register to test esm code (default npm run test behavior)
• added steps to test cjs build with tsc (in ci esm and cjs both should be tested)
• removed tsup from package dev dependencies

this change improves test organization, stability, and enhances testing capabilities without increasing the binary size.

[mikemaccana]

Glad that adding jest won't increase the distribution size, I just don't see the need to add jest as a dev dependency. Asides from jest not needing to exist - this repo already has a test suite using node's native tools, jest has a bad reputation (particularly in the node space) for creating hard to debug edge cases since it monkey patches various libraries.

This is a good PR otherwise - I'd just move the tests into their own folder, without adding jest.