[Feature] Conditional splitAttributes

Proposals:

• "xml.format.splitAttributesLongLines": boolean - Only split attributes when the line is longer than xml.format.maxLineWidth. Probably easier to implement than...
• "xml.format.groupAttributesNamespaces": string[][] - An array of string arrays. Each string array is a set of attribute namespaces to group.

Another addition for folks editing XAML and derived languages. This would allow vscode-xml to format more similarly to Visual Studio's XAML formatting. That is...

• The first attribute stays on the same line as its XML tag.
• Subsequent attributes are split onto separate lines and then aligned to the first attribute.
• Certain attributes are grouped on one line e.g. mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450".
  • A. Split when line would otherwise be longer than xml.format.maxLineWidth. This "dumb" solution is easier to implement than...
  • B. Try grouping attributes if they come from specified namespaces. For example, if the user configured grouping attributes from namespaces http://schemas.microsoft.com/expression/blend/2008 (usually xmlns:d) and http://schemas.openxmlformats.org/markup-compatibility/2006 (usually xmlns:mc), the formatter should not split attributes from those namespaces unless the line would be too long.

I'm unsure if the logic behind apparent "attribute grouping" in Visual Studio is specific to XAML grammar or if it's a simple max-line-length rule.

Example configuration for xml.format.groupAttributesNamespaces

[
  [
    "http://schemas.microsoft.com/expression/blend/2008",
     "http://schemas.openxmlformats.org/markup-compatibility/2006"
  ],
  [
    "namepsaceA",
	"namespaceB",
	"namespaceC"
]

No implementation files, tests, or entry points are named. First resolve whether the feature should use max-line-width logic, namespace-based grouping, or both; done means the chosen configuration formats XAML-style attributes as specified without exceeding the configured line width.