Handle replaceStart/replaceEnd in completion proposal

Fixes microsoft#524
Ensures completion text can be appended by clients

E.g.

For `com.|` the completion proposal has:

    completion: com.sun.jndi.ldap.pool
    completionLocation: 3
    replaceEnd: 4
    replaceStart: 0

`insertText` will be `sun.jndi.ldap.pool`

For `List.|` the completion proposal has:

    completion: of()
    completionLocation: 4
    replaceStart: 5
    replaceEnd: 5

`insertText` will be `of()`

com.microsoft.java.debug.plugin/src/main/java/com/microsoft/java/debug/plugin/internal/CompletionProposalRequestor.java

@@ -160,9 +160,16 @@ public CompletionItem toCompletionItem(CompletionProposal proposal, int index) {
         data.put(CompletionResolveHandler.DATA_FIELD_REQUEST_ID, String.valueOf(response.getId()));
         data.put(CompletionResolveHandler.DATA_FIELD_PROPOSAL_ID, String.valueOf(index));
         $.setData(data);
+
         this.descriptionProvider.updateDescription(proposal, $);
         // Use fully qualified name as needed.
-        $.setInsertText(String.valueOf(proposal.getCompletion()));
+        String insertText = String.valueOf(proposal.getCompletion());
+        int suffix = proposal.getReplaceEnd() - proposal.getReplaceStart();
+        if (suffix > 0) {
+          $.setInsertText(insertText.substring(suffix));
+        } else {
+          $.setInsertText(insertText);
+        }
         adjustCompleteItem($);
         $.setSortText(SortTextHelper.computeSortText(proposal));
         return $;