Add `codeql` executable to PATH of VS Code Terminal

Start in extension.ts at the activate function and review VS Code's environmentVariableCollection support. Compare the proof of concept with the open questions about PATH ordering, CLI updates, custom paths, and terminal activation; done means the intended behavior is defined and codeql is available in supported VS Code terminals.

Is your feature request related to a problem? Please describe.
In some cases it is still necessary to use the CodeQL CLI, such as for creating a database (see also #306). If you don't want to manually install the CodeQL CLI but instead want to rely on the extension installing the latest version, this is a bit cumbersome because the executable is not by default available on the PATH, and its directory name (the distributionX part) changes after every CodeQL CLI update.

Describe the solution you'd like
At least the extension could make the codeql executable available in the VS Code Terminal, see for example https://code.visualstudio.com/updates/v1_46#_environment-variable-collection, and possibly related https://github.com/microsoft/vscode-python/issues/11039 (might contain useful information and links).

Open questions:

• Should this be opt-in / opt-out (or always active without being possible to deactivate)?
• Should it prepend or append the CodeQL CLI directory to the PATH?
  (would make a difference if the user has already another codeql executable on the PATH)
• Will this correctly handle CodeQL CLI updates performed by the extension (automatic or manually triggered)?
  (or will codeql not be available in all open terminals afterwards anymore)
• Will this correctly handle setting a custom CodeQL CLI path in the settings while the extension is running?
  (and what is the 'correct' behavior? maybe it is desired that already open Terminals keep using the previously used CodeQL CLI)
• Will codeql be available in the Terminal even if the extension hasn't been activated yet?
  (looks like this is the case if the extension had been activated previously when VS Code was open; not sure why this works)

As proof of concept I added locally the following to the activate function in extension.ts:

const collection = ctx.environmentVariableCollection;
const separator = process.platform === "win32" ? ";" : ":";
collection.prepend(
  "PATH",
  `C:\\Users\\user\\AppData\\Roaming\\Code\\User\\globalStorage\\github.vscode-codeql\\distribution16\\codeql${separator}`,
);

And that seems to work, making codeql available in the VS Code Terminal (if opened after the extension has been activated).

Describe alternatives you've considered

• Let the extension add the CodeQL CLI globally to the PATH, so it is available even outside of VS Code
  Probably not easily possible in a portable way, might make it difficult to update the PATH when the CodeQL CLI is updated, and might also not properly update the PATH when the extension is uninstalled.
• The user has to manually install CodeQL CLI, add it to the PATH and specify it in the extension settings
  They won't receive automatic updates this way though.