title: Adding conditions to "when" from VS-code extension?
body: <p>The code editor I've been using for the past 20 years (codewright) allows you to set a "select mode". When that is set, all keyboard cursor movements extend the selection. In VS Code, you can extend the selection by holding down the shift key (for example, Shift down-arrow), but I am looking for a way to do that without the shift key.</p>
  
  <p>I have written an extension that mostly does it, but I would have had to do far less work if I could have created a new condition for the "when" clause in <code>keybindings.json</code>. For example, I would have liked to change </p>
  
  <pre class="lang-json prettyprint-override"><code>{ "key": "shift+down",    "command": "cursorDownSelect",
                            "when": "editorTextFocus" },
  </code></pre>
  
  <p>to something like</p>
  
  <pre class="lang-json prettyprint-override"><code>{ "key": "down",    "command": "cursorDownSelect",
                      "when": "editorTextFocus || extensionSelectionMode" },
  { "key": "down",    "command": "cursorDown",
                      "when": "editorTextFocus" },
  </code></pre>
  
  <p>Is there a way to do add a such a condition from an extension?</p>