Switch to ESM

Is your enhancement related to a problem? Please describe.

Currently yaml-language-server is distributed as:

• CommonJS
• UMD
• Faux ESM

These are all outdated practices that aim to provide backwards compatibility for programmatic use. But in reality, they make the project harder to work with.

Describe the solution you would like

• Go ESM only
• Drop CommonJS support
• Drop UMD support
• Drop Node.js 20 support
• Replace Mocha with node:test

Describe alternatives you have considered

• The use of ts-node will likely be problematic. This is why I suggest to use Node.js type-stripping.
• I don’t know how well Mocha works with Node.js type-stripping. This is why I suggest replace Mocha with node:test and drop support for Node.js 20. An alternative would be to build the tests and running its build output.

Additional context

The current state of JavaScript modules is as follows:

• Node.js 12 added support for ESM.
• Node.js 20 added support for require(esm).
• Node.js 22 added support for type-stripping.
• TypeScript 6 is going to remove support for UMD.
• VSCode plans to move to ESM for its client libraries (https://github.com/microsoft/vscode/issues/192144#issuecomment-3595706764).
• The upcoming version 3 of the yaml package will be ESM only (https://github.com/eemeli/yaml/issues/601).
• yaml-language-server is tested against Node.js 20.
• At the moment of writing, Node.js 20 is about to reach EOL.

No files or test paths are named. Begin by locating the current CommonJS, UMD, and ESM entry points and the Mocha-based test setup, then review how TypeScript, ts-node, and Node.js type-stripping are used; done means ESM-only distribution and tests running with node:test.