Switch to ESM and add native TypeScript task loading (Node 22.18+), update tests and docs

[benjaie]

Motivation

• Modernize the package to be published as ESM, leverage Node's native type-stripping for erasable TypeScript, and make .ts/.mts first-class task file extensions.
• Increase minimum Node version support to a current LTS (Node 22.18+) to rely on native ESM/TypeScript features and avoid older Node issues.
• Update examples, docs, and tests to match ESM-style usage and ensure loader behavior is deterministic for both ESM and legacy CommonJS consumers.

Description

• Mark the package ESM in docs and release notes and document breaking changes (ESM package, .js tasks in ESM projects must use export default, and .ts/.mts are supported by default).
• Add .ts and .mts to the default fileExtensions in src/config.ts, src/index.ts docs, LoadTaskFromJsPlugin.ts defaults, and various doc examples and performance presets.
• Convert many example task files and README snippets to ESM-style import and export default and add TS/MTS fixtures (__tests__/fixtures-esm/tasks/typescript.ts and module-typescript.mts).
• Update tests to use import type for type-only imports where appropriate and add assertions that .ts/.mts tasks are discovered and executed.
• Rename jest.config.js to jest.config.cjs and configure Jest to treat .ts as ESM via extensionsToTreatAsEsm: [".ts"].
• Update tsconfig.json to enable verbatimModuleSyntax to better preserve ESM semantics during compilation.
• Update RELEASE_NOTES.md, docs under website/docs/ and contributing guides to explain ESM/TS changes and Node requirements.

Testing

• Ran the TypeScript build (yarn prepack) to ensure compilation and type output succeed, and it completed successfully.
• Ran the unit test suite (yarn test / jest) including the updated fixtures and tests verifying .ts/.mts task loading, and all tests passed.

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