Fix: Specifying custom style sheets with a relative path does not work

[hirschmann]

Setting relative paths in markdown-pdf.styles currently (VS Code 1.41.0, Markdown PDF 1.4.1) does not work because the path will always be converted to a absolute path.
For example styles/custom.css will become files:///styles/custom.css
This makes Markdown PDF fall back to the VS Code default styles.

The reason is, that hrefUri.scheme === 'file' is not only true for absolute file paths, but also for relative paths.

This PR fixes this issue.

[JamesAlias]

As dirty workaround while we wait for this PR to be merged and released:
You can include the style file like this:

<!-- styles.md -->
<style>
  ... content of your style file
</style>

<!-- in your markdownThatWillBeRenderedToPDF.md -->
:[styles](<path/to/styles.md>)
... content to be rendered to PDF

The styles will then be included as style tag in the transient html file. (md -> html -> pdf).

[teterkin]

JS is correct.

[yzane]

Thanks for your PR!