Allow multi-select in query history view

[aeisenberg]

Some commands in the query history view make sense to support multiple items:

• codeQLQueryHistory.removeHistoryItem --- we should be able to remove multiple items at once
• codeQLQueryHistory.compareWith --- if exactly two items are selected, the compare view should focus on the two of them (if they are compatible queries)

I would like to ensure that when there is multi-select, only the appropriate commands are active, but due to microsoft/vscode#99767, this feature has not been implemented in vscode. So, we will just have to throw an error if the wrong number of items are selected for a query.