Upgrade TypeScript (and related) to 5.0, and start using package.json conditional exports

[milesrichardson]

Somewhat surprisingly, this all seems to be working.

Basically:

• Upgrade TypeScript to latest
• Opt out of verbatimModuleSyntax (but continue using isolatedModules: true)
• Upgrade tsc-multi to latest get error with typescript 5 tommy351/tsc-multi#19
• Continue targeting CJS outputs! :)
• Use exports field of package.json, and specifically "conditional exports," so that published modules point to CJS or ESM (only using es2020 for ESM atm, although we build esnext) based on consumer, e.g.:

// print out the resolved path of a module imported via the module system
// (this flag is just necessary to get import.meta.resolve to print out the file)
// node --experimental-import-meta-resolve blah.mjs
console.log(await import.meta.resolve("@madatdata/react"));

// outputs:
// file:///path/to/madatdata/examples/react-nextjs-basic-hooks/node_modules/@madatdata/react/build/es2020/index.mjs

and

// node blah.js
console.log(require.resolve("@madatdata/react"));

// outputs:
// /path/to/madatdata/examples/react-nextjs-basic-hooks/node_modules/@madatdata/react/build/es2020-commonjs/index.cjs

Each package.json now includes this:

{
"exports": {
    ".": {
      "dev": "./index.ts", // <---- used by tsc customConditions and vitest resolve.conditions
      "require": {
        "types": "./build/es2020-commonjs/index.d.ts",
        "default": "./build/es2020-commonjs/index.cjs"
      },
      "import": {
        "types": "./build/es2020/index.d.ts",
        "default": "./build/es2020/index.mjs"
      },
      "default": {
        "types": "./build/es2020/index.d.ts",
        "default": "./build/es2020/index.mjs"
      }
    }
  }
}

• Use a custom conditional export called dev which is not consumed by external tools (and therefore will not interfere with downstream packages), and use the new TypeScript 5.0 feature customConditions to make sure that TypeScript can resolve using moduleResolution: nodenext by seeing index.ts (instead of something in ./build for each package). Similarly, use a config flag resolutions.conditions in vitest, which passes it to vite, which passes it to node --conditions (see: Vitest doesn't handle package.json exports conditions like browser vitest-dev/vitest#2603 (comment))
• Upgrade vitest to latest
• Upgrade msw to latest and drop our hacky patch
• Build latest versions of all packages and consume them with examples to make sure they still work

Once this is merged I'll publish the latest versions to npm.

Unfortunately I think skypack will still be broken, since it doesn't install dependencies of a package. But we'll see, maybe it will magically work too (doubt it).