feat: auto config preset

[danielroe]

This PR adds support for magic inferring of build configuration from package.json, if it isn't specified in build.config.

For example:

{
  "exports": "./dist/index.mjs",
  "types": "dist/index.d.ts",
  "bin": {
    "unbuild": "./dist/cli.mjs"
  }
}

Assuming we have the appropriate source files, unbuild will infer that we should generate types, that we don't need to emit CJS outputs, and that there are two entries: src/index and src/cli. We also support generating mkdist-powered directory outputs from export subpath patterns.

It was inspired by siroc's implementation but there are some important differences. Notably we can't define on a per-entrypoint whether or not to emit declaration file or CJS, so if any one output requires .cjs or .d.ts then it will switch on the flag for the entire build.

TODO:

• add validation for built files in subsequent PR: validate module fields #6
• add logging to tell user what has been inferred

resolves #3