[html] code complete for href anchors

<!-- Please search existing issues to avoid creating duplicates. --> <!-- Also please test using the latest insiders build to make sure your issue has not already been fixed: https://code.visualstudio.com/insiders/ --> <!-- Use Help > Report Issue to prefill these. --> - VSCode Version: Version: 1.30.1 (user setup) Commit: dea8705087adb1b5e5ae1d9123278e178656186a Date: 2018-12-18T18:12:07.165Z Electron: 2.0.12 Chrome: 61.0.3163.100 Node.js: 8.9.3 V8: 6.1.534.41 - OS Version: Windows_NT x64 10.0.17134 Steps to Reproduce: 1. Create html file and some elements with their own id like so: `<div id="TEXTHERE">` 2. Create an `<a>` tag with href like this: `<a href="#TEXTHERE>content</a>` 3. You will see no autocompletion for the href. If you reload VSCode fresh the autocompletion works for a few secs (via word autocompletion) but stops to work when VSCode has completed all startup processes. Use-case: You have a static HTML file with a lot of elements with IDs and want to check while typing in an anchor link if that ID exists. I tried three different autocompletion plugins, none of them could give me this functionality. This works flawlessly in Atom, though. <!-- Launch with `code --disable-extensions` to check. --> Does this issue occur when all extensions are disabled?: **Yes**