Add copy-custom-fonts parameter to HtmlConfig to enable custom fonts

[emirror-de]

This PR introduces a new parameter to the HtmlConfig struct named copy_custom_fonts. When set to true, the fonts/fonts.css file is scanned for @font-face rules that contain a src: url(font-file.name) attribute. The files that have been found are copied into the book folder on build/serve.

If copy_fonts is also enabled, the build in fonts/fonts.css is extended by the custom one. In addition to that, the build in font files are also copied to the output folder.

Example usage:

Create a fonts/ folder in your book root. Then copy the fonts you want to use into this folder (in this example we are assuming Lato-Regular.ttf)

Create a custom fonts file fonts/fonts.css

@font-face {
     font-family: "Lato";
     font-style: normal;
     font-weight: normal;
     src: url('Lato-Regular.ttf');
}

Setup your book.toml that it contains the following parameters:

[output.html]
copy-fonts = true
copy-custom-fonts = true

Adjust your theme/css/general.css file according to your needs, for example:

html {
    font-family: "Lato", sans-serif;
}

It adds a new dependency lewp-css to Cargo.toml for simple parsing of the fonts/fonts.css file.
It also updates the msrv to 1.56.0 because lewp-css requires edition 2021.
Furthermore, the clap dependency gets updated.