snowman build should be able to build only static files when given the static flag

[Abbe98]

snowman build --static should build only the static files because as a developer I often work on issues only affecting client-side CSS and JavaScript and I want to avoid long build times when I work on large sites. Part of #2.

Implementation notes

While the current implementation just deletes and copies the static files as-is we cant do the same here. Because a file might have been deleted we need to be able to remember past files so that we can selective delete them if needed.

1. when copying static files store the result in .snowman/static_history.txt
2. delete the files given in static_history.txt
3. copy all static files and update static_history.txt

These features should be in a non-cmd package to ensure they can be used in #2.

[Abbe98]

Following 401e835 we now have helper functions for reading and writing line separated files.

https://github.com/glaciers-in-archives/snowman/blob/main/internal/utils/utils.go#L58
https://github.com/glaciers-in-archives/snowman/blob/main/internal/utils/utils.go#L80

[Abbe98]

We should move the CopyDir function into a static package while adding the history functionality. There should also be a separate function for clearing cache items based on the history.

snowman/internal/utils/utils.go

Line 17 in 401e835

func CopyDir(from string, to string) error {