Add an empty query cell above the first one

[prrao87]

When the user deletes all empty query cells using the ❌ button, there's no way to add a new cell other than refreshing the page (this might cause them to lose any existing queries). I think it makes sense to add a small box at the top of the page called "Insert new cell" similar to how it would be in a Jupyter notebook or the Neo4j query editor. Something like the following:

[mewim]

The design was that there should always be at least one cell.

https://github.com/kuzudb/explorer/blob/f988aff1c7fcb104654bec69f896f64ae2cbbbf5/src/components/ShellView/ShellMainView.vue#L83

If there is 0 cell, an empty cell will be created automatically.