xml.format.emptyElements should respect grammar constraint

[angelozerr]

Please read #492 (comment)

XML formatting should give a preferences like xml.format.autoTagClose to auto close the element which is formatted.

In other words, formatting :

<element name="myName"></element>

should format like this:

<element name="myName" />

[fbricon]

This might be trickier than it seems: it should respect XSD's nillable=false definitions

[angelozerr]

This issue is managed by xml.format.emptyElements in old formatter but don't take care of XSD/DTD constraints (it should respect XSD's nillable=false definitions)

@JessicaJHee is supporting xml.format.emptyElements in experimental formatter in #1297 and the experimental formatter can access to grammar information when https://github.com/redhat-developer/vscode-xml/blob/main/docs/Formatting.md#xmlformatgrammarawareformatting i sset to true.

@JessicaJHee once #1297 will be merged, I think it could be interesting to use grammar information like it is done for format element category at https://github.com/eclipse/lemminx/blob/f4d00267bb82221eb3b8d281b616d467940d8d36/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/ContentModelFormatterParticipant.java#L50

The idea is to check if the element to format can be format to empty element according the XSD/DTD constraints by using https://github.com/eclipse/lemminx/blob/f4d00267bb82221eb3b8d281b616d467940d8d36/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/xsd/contentmodel/CMXSDElementDeclaration.java#L461