`FluentGlyphRangeBuilder` can build glyph ranges without having to
allocate native ImGui objects, and supports adding `ReadOnlySpan` and
`IEnumerable` of `char`s and `byte`s, `UnicodeRange`s, and known glyph
ranges for select cultures (CJK for now.)
Added `IFontAtlasBuildToolkitPreBuild.AttachWindowsDefaultFont` which
has a predefined list of default Windows fonts per language.
`PostPromotion` is removed, as `IFontHandle.ImFontChanged` now does the job. It also removes the possibility that resources may get disposed while post promotion callback is in progress.
* `IFontHandle.ImFontChanged` is now called with a locked instance of the font.
* `IFontHandle.ImFontLocked`: Added `NewRef` to increase reference count.
