Full support for the CSS `white-space` property

The CSS `white-space` property is defined in sections 3 and 4 of the css-text-4 specification: https://drafts.csswg.org/css-text-4/#white-space-property. It is a very important property for accurately reproducing how browsers lay out text. And can also be used to account for differences between desired behaviour between editable and non-editable text around how whitespace is "hung" at the end of lines. It is considered a "shorthand" property that decomposes into the following longhands: #### `text-wrap-mode` This property has two variants: `wrap` and `nowrap`. `nowrap` disables wrapping entirely. ```rust enum TextWrapMode { Wrap, NoWrap, } ``` Status: fully implemented. #### `white-space-collapse` This property controls: - How whitespace is collapsed - How whitespace at the end of the line is "hung" (or not hung) ```rust enum WhiteSpaceCollapse { Collapse, Discard, Preserve, PreserveBreaks, PreserveSpaces, BreakSpaces, } ``` See spec (https://drafts.csswg.org/css-text-4/#white-space-collapsing) for the meanings of each variant. Status: we currently [implement](https://docs.rs/parley/latest/parley/style/enum.WhiteSpaceCollapse.html) only the `Collapse` and `Preserve` variants in a somewhat hacky way [in the `TreeBuilder`](https://docs.rs/parley/latest/parley/struct.TreeBuilder.html#method.set_white_space_mode), and we also only implement the "white space collapsing" behaviour, and do not implement the different "whitespace hanging" behaviours (implementing the latter will require us to pass `WhiteSpaceCollapse` into layout as a style which we do not currently do). See also: discussion on https://github.com/linebender/parley/pull/485 which attempted fixes for whitespace hanging behaviour. #### `white-space-trim` Controls "whitespace trimming" at the beggining and end of spans. ```rust enum WhiteSpaceCollapse { DiscardBefore, DiscardAfter, DiscardInner, } ``` Status: some basic trimming is implemented hackily in the `TreeBuilder`, but there are bugs in the current implementation. And it also doesn't allow full control over the trimming using a style.