This commit increases the test coverage for the `pkg/datanode` package
from 58.1% to 66.9%.
The following changes were made:
- Added a new test file, `pkg/datanode/fromtar_test.go`, to test the
`FromTar` function.
- Added a new test file, `pkg/datanode/fs_test.go`, to test the `Stat`,
`Exists`, and `Walk` functions.
- Refactored the existing tests in `pkg/datanode/datanode_test.go` to
remove the monolithic test function and improve the organization of
the tests.
This commit introduces a new `DataNode` package, which provides an in-memory, `fs.FS`-compatible filesystem with a `debme`-like interface. The `DataNode` can be serialized to and from a TAR archive, making it suitable for storing downloaded assets.
The `pwa` and `serve` commands have been refactored to use the `DataNode`. The `pwa` command now packages downloaded PWA assets into a `DataNode` and saves it as a `.dat` file. The `serve` command loads a `.dat` file into a `DataNode` and serves its contents.
