No way to call FileOperationPattern.setMatches() to only match files or folders

[testforstephen]

When i want to register FileOperationsServerCapabilities to enable willRenameFiles support, i found it's not possible to create a FileOperationPattern to only match files or folders.

The setter of the field FileOperationPatternKind matches asks for an instance of FileOperationPatternKind. See https://github.com/eclipse/lsp4j/blob/4901031aa9fb277711b68a2a8dc0a234fab41b5f/org.eclipse.lsp4j/src/main/java/org/eclipse/lsp4j/Protocol.xtend#L7036-L7058

But the class FileOperationPatternKind hides its constructor and no way to create an instance for it. See
https://github.com/eclipse/lsp4j/blob/4901031aa9fb277711b68a2a8dc0a234fab41b5f/org.eclipse.lsp4j/src/main/java/org/eclipse/lsp4j/FileOperationPatternKind.java#L20-L33

We should make the matches field to be String type, and accept FileOperationPatternKind.File and FileOperationPatternKind.Folder as value.

A fix could be:

class FileOperationPattern {

    	/**
	 * Whether to match files or folders with this pattern.
	 *
	 * The match kind could be <code>FileOperationPatternKind.File</code>, <code>FileOperationPatternKind.Folder</code>.
	 *
	 * Matches both if undefined.
	 */
	String matches

}