Vanillaplusjs

Resolves several issues leading to the dependency graph slowly getting corrupted. Specifically:

- Fixed scanning css files with relative urls marking the input file as a child, rather than the required hash file
- Fixed scanning css files with inline image data not properly marking the produced files
- Fixed losing dependencies when rebuilding a file that wasn't scanned but depended on a file produced by a file which also wasn't scanned

This adds a prompt when doing `vanillaplusjs clean` to clear out generated placeholder files with information on the consequences. The prompt can be suppressed by providing an answer with the arguments `--placeholders` or `--no-placeholders` respectively.

This also improves dependency flows css url-linking and html link refs to select the file which produces the required `.hash` file, rather than assuming that `foo.hash` is always generated by a corresponding `foo` source file, which is not true e.g., when using the icon generator

This release fixes an issue where placeholder files were not correctly considered as implicitly depending on the generating file. In particular, when using the image glue file to generate `foo.images.js` from `foo.images.json`, the produced hash file `foo.images.js.hash` would not be produced until `foo.images.json` was processed. So if `other.js` imports from `foo.images.js` which requires the `foo.images.js.hash`, it would be a race for if `foo.images.json` happens to finish processing before `foo.images.js` attempts to read `foo.images.js.hash`

This update maintains a placeholder graph which is used to correctly mark `other.js` as depending on `foo.images.json` in the above example.

This update improves the reliability of creating directories in parallel on windows by adding a test if the directory exists before attempting to create it (and catching errors), and retrying permission errors with exponential back-off up to 5 times. This is particularly helpful on networked drives, though it can impact any large project.

Resolves some minor dev inconveniences:

- When freshly building a project which had external files (as if by clean then build), an error would be raised that the `out` folder did not exist
- When building on windows with many cores, sometimes when deleting a folder concurrently, instead of `FileNotFoundError`, a `PermissionError` is raised, which would cause a crash.

This release adds automatic hash suffixing to all URLs in CSS, so e.g.

css
font-face {
font-family: "Test";
src: url("/assets/fonts/test.ttf");
font-weight: normal;
font-style: normal;
}


will become

css
font-face {
font-family: "Test";
src: url("/assets/fonts/test.ttf?v=QUVT2uNMmAwpHxaQ4w6wrDdwddIgvIWPbLF0X9wZKjs%3D&pv=1.2.3");
font-weight: normal;
font-style: normal;
}


which ensures that the preloaded file, when using preloading, is a match for the actual font file