Add support for SnippetTextEdit in code actions

[hopehadfield]

Fixes https://github.com/redhat-developer/vscode-java/issues/3686

Requires https://github.com/eclipse-jdtls/eclipse.jdt.ls/pull/3229

[rgrunber]

Random fact learned : CodeAction.set(Uri, null) will clear the edits for a particular Uri but a non-null 2nd argument will merely append. See https://github.com/microsoft/vscode/issues/197315