Preferences to enable/disable symbol provider

[angelozerr]

With large file like nasa.xml, the vscode outline doesn't work. It seems lsp4j doesn't support a big symbol provider:

[Trace - 17:48:08] Received response 'textDocument/documentSymbol - (1)' in 6234ms. Request failed: Internal error. (-32603).
[Error - 17:48:08] Request textDocument/documentSymbol failed.
  Message: Internal error.
  Code: -32603 
java.util.concurrent.CompletionException: java.lang.OutOfMemoryError: Java heap space
	at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
	at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
	at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:659)
	at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:632)
	at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:474)
	at java.util.concurrent.CompletableFuture.postFire(CompletableFuture.java:561)
	at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:580)
	at java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:443)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
Caused by: java.lang.OutOfMemoryError: Java heap space
	at java.lang.StringCoding$StringEncoder.encode(StringCoding.java:300)
	at java.lang.StringCoding.encode(StringCoding.java:344)
	at java.lang.String.getBytes(String.java:918)
	at org.eclipse.lsp4j.jsonrpc.json.StreamMessageConsumer.consume(StreamMessageConsumer.java:60)
	at org.eclipse.lsp4j.jsonrpc.RemoteEndpoint.lambda$handleRequest$1(RemoteEndpoint.java:281)
	at org.eclipse.lsp4j.jsonrpc.RemoteEndpoint$$Lambda$17/1028064261.accept(Unknown Source)
	at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:656)
	... 9 more

To fix this problem, we need to provide a preferences to enable/disable symbol provider.

In the future we could provide a new preferences to set the number of symbol returned by the symbol provider. I started to do that in https://github.com/angelozerr/lsp4xml/blob/7bca4bf80031cbcd522eaf41f3ba217ceb942bfb/org.eclipse.lsp4xml/src/main/java/org/eclipse/lsp4xml/services/XMLSymbolsProvider.java#L67