[html] handle HTML-escaped characters in links

Issue Type: <b>Bug</b> Given trivial HTML or XML content that contains a url with a valid entity escape code: ```xml <a href="https://www.example.com/foo.html?bar&amp;baz">Link</a> ``` **Expected behavior** The clickable hyperlink directs the user to `https://www.example.com/foo.html?bar&baz` **Actual behavior** The clickable hyperlink directs the user to a corrupted version of the url. The type of corruption differs between XML and HTML document contexts. _In an XML context_, `&amp;` is left verbatim and the clickable link directs the user to `https://www.example.com/foo.html?bar&amp;baz`. _In an HTML context,_ the `&` in the `&amp;` HTML entity is double-escaped using percentage encoding and the user is directed to` https://www.example.com/foo.html?bar%26amp%3Bbaz`. **Impact** Not just limited to HTML or XML documents. Can confirm that among other things, this _also_ breaks `<seealso href="..." />` type links in C# XMLDoc. Both the in-document clickable link _and_ the link rendered in the popup documentation tooltip. ----- VS Code version: Code 1.52.1 (ea3859d4ba2f3e577a159bc91e3074c5d85c0523, 2020-12-16T16:34:46.910Z) OS version: Windows_NT x64 10.0.18363 <details> <summary>System Info</summary> |Item|Value| |---|---| |CPUs|Intel(R) Xeon(R) CPU E5-1603 v4 @ 2.80GHz (4 x 2794)| |GPU Status|2d_canvas: enabled<br>flash_3d: enabled<br>flash_stage3d: enabled<br>flash_stage3d_baseline: enabled<br>gpu_compositing: enabled<br>multiple_raster_threads: enabled_on<br>oop_rasterization: disabled_off<br>opengl: enabled_on<br>protected_video_decode: unavailable_off<br>rasterization: enabled<br>skia_renderer: disabled_off_ok<br>video_decode: enabled<br>vulkan: disabled_off<br>webgl: enabled<br>webgl2: enabled| |Load (avg)|undefined| |Memory (System)|15.92GB (4.23GB free)| |Process Argv|--crash-reporter-id 3bd078e3-b0bf-4d5c-a65c-a246d819c6d9| |Screen Reader|no| |VM|0%| </details><details><summary>Extensions (6)</summary> Extension|Author (truncated)|Version ---|---|--- alignmen…