[Feature]: stabilize config design

[fi3ework]

What problem does this feature solve?

Stabilize config design schema for 1.0, ensure no breaking change in foreseeable future.

What does the proposed API look like?

x

[Timeless0911]

TBD: Remove required limitation of lib field which means we can generate pure esm package with zero configuration.

[Timeless0911]

TBD: Simplify source.entry configuration in bundleless mode.

current

export default defineConfig({
  source: {
    entry: {
      index: ['src', '!src/**/*.test.ts'],
    },
  },
});

expect

export default defineConfig({
  source: {
    entry: ["src", "!src/**/*.test.ts"],
  }
});

[Timeless0911]

TBD: support web-worker target.

https://github.com/web-infra-dev/rslib/issues/929

Also, neutral target?