Update custom whitespace API

[cmyr]

The old API was designed to work around the fact that the plist and quick-xml crates had different APIs for specifying custom indent behaviour. The plist crate has since been updated to match the API of quick-xml, which means that we can do the same.

[cmyr]

@RickyDaMa feel free to give this a review if you like! I'm going to hold the release a day because I think we can get a release of the plist crate done, and then we can update quick-xml.

[RickyDaMa]

Looks good to me! Releasing later is fine, but in time for Thursday would be preferable for me

The only thing about this PR that gives me pause is the paths of the constants WriteOptions::SPACE doesn't seem quite right - it doesn't read intuitively, you have to look at the documentation.

You could maybe do something like

pub mod indent_chars {
    /// The ASCII value of the space character (0x20)
    pub const SPACE: u8 = b' ';
    /// The ASCII value of the tab character (0x09)
    pub const TAB: u8 = b'\t';
}

so that the path is indent_chars::SPACE instead, but it's a tiny nitpick

[cmyr]

My desire was to not require an additional namespace or import for these, and so they're provided here as a convenience. In general I think the pattern of defining constants on the type where they're used is at least modestly idiomatic? I could get fancy and define a custom enum or something but I think that's overthinking a fairly niche feature. :)