[native_assets_cli] Drop Config suffixes v1

[dcharkes]

Addresses:

• [native_assets_cli] Move BuildConfig.targetOS to BuildConfig.codeConfig.targetOS, remove it entirely or extend to cover web #1738 (comment)

A variant of:

• [native_assets_cli] Drop Config suffixes v2 #1830

This PR normalizes the code and data specific APIs on the config, config builder, output and output builders under a .code and .data extension. Except for the assets, the are available under config/output.assets.code and config/output.assets.data.

Example link hook code:

void main(List<String> args) async {
  await link(
    args,
    (config, output) async =>
        output.data.addAssets(treeshake(config.assets.data)),
  );
}

Example build hook code:

void main(List<String> arguments) async {
  await build(arguments, (config, output) async {
    if (config.code.targetOS == OS.android) {
      config.code.android.targetNdkApi;
    }

    output.code.addAsset( ... );
  });
}

This drops Config suffixes from the various config accessors.
This changes output.codeAssets.add( to output.code.addAsset(.

The weird quirk in this PR is that the list of assets per type is available under config.assets.code while all the other code related things are available under config.code and output.code. To address this see:

• [native_assets_cli] Drop Config suffixes v2 #1830

[coveralls]

coverage: 88.07% (+0.1%) from 87.965%
when pulling c9c6362 on config-names-v1
into 8c16b6c on main.

[dcharkes]

#1738 (comment)