Ephemeral quick query editor

[p0]

Is your feature request related to a problem? Please describe.
When we added the quick query feature in #210, we had to stick within the confines of what was possible in VS Code at the time. This made the result a little less slick-feeling than the equivalent QL4E functionality: Quick queries are genuine files on disk, but in an obscure directory; this means they are stateful, and don't interact as well with repeated quick-query executions as QL4E's ephemeral design.

Describe the solution you'd like
With the advent of webview views, I think it's now possible do get feature-parity with Eclipse. We could create a "quick query" view which effectively consists of a large textarea that does CodeQL error checking and syntax highlighting in the context of the active database and workspace (meaning that the active database determines the language, and the qlpacks from the workspace are on the search path, like normal). We can then execute quick queries by passing the content of that text area to the query server, rather than by writing it to disk and passing a path.

The main benefit this unlocks is in conjunction with the query history view: If I double-click on a query execution that corresponds to a quick query, I want the quick query view to open, populated with the QL code that corresponds to the execution I double-clicked on. (We have support for looking at the QL text of existing query executions, but that goes via a slightly awkward readonly editor.)

A side benefit is the removal of the hidden quick-queries directory deep in the workspace, and any pretense that quick queries live in the filesystem, should appear in the outline, should be allowed to have QLLs next to them, and so on.

[alexet]

We can't have a vscode editor inside a webview so we can't get nice text editing without implementing a web based text editor (or at least using one). We could have done this all inside a webview already. I don't think the location of where we could put the webview was the problem at the time. I think that without ide features it would be pretty painful.

[sj]

Just in case anyone is looking for the "Quick query" functionality as it exists today in the VS Code extension: https://codeql.github.com/docs/codeql-for-visual-studio-code/analyzing-your-projects/#running-a-quick-query