Support for output search

This is the first pass at search which can search output in a generic
way. This commit does this on a best effort through walking and
manipulating the DOM. There are limitations, including the inability to
handle that to a user <span><b>foo</b>bar</span> should be searched as
foobar. This also will not handle SVGs and does not expose a way to
delegate search to output presenters. Still, this works very well on
stdout/stderr type messages, tables, etc.

Fixes: #6768

packages/documentsearch/package.json

@@ -44,6 +44,7 @@
     "@phosphor/signaling": "^1.3.0",
     "@phosphor/widgets": "^1.9.0",
     "codemirror": "~5.47.0",
+    "lodash": "~4.17.15",
     "react": "~16.8.4"
   },
   "devDependencies": {

packages/documentsearch/src/providers/codemirrorsearchprovider.ts

@@ -83,8 +83,6 @@ export class CodeMirrorSearchProvider
     // canSearchOn is a type guard that guarantees the type of .editor
     this._cm = searchTarget.content.editor;
     return this._startQuery(query);
-
-    throw new Error('Cannot find Codemirror instance to search');
   }
 
   /**
@@ -300,6 +298,9 @@ export class CodeMirrorSearchProvider
     return null;
   }
 
+  get editor(): CodeMirrorEditor {
+    return this._cm;
+  }
   /**
    * Set whether or not the CodemirrorSearchProvider will wrap to the beginning
    * or end of the document on invocations of highlightNext or highlightPrevious, respectively