[html] handle HTML-escaped characters in JavaScript attribute properties

Issue Type: <b>Bug</b> - Create an HTML attribute with JS in it, such as a button `onclick` event. - Since attributes cannot use raw double quotes (Already used to open/close the attribute, HTML will give a syntax error), use `&quot;`. - VSC will incorrectly mark it as an error **Example** ```<button onclick="someFunction(&quot;string&quot;);">``` VS Code version: Code 1.51.1 (e5a624b788d92b8d34d1392e4c4d9789406efe8f, 2020-11-10T23:34:32.027Z) OS version: Windows_NT x64 10.0.18363 <details> <summary>System Info</summary> |Item|Value| |---|---| |CPUs|Intel(R) Core(TM) i5-9400F CPU @ 2.90GHz (6 x 2904)| |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)|15.93GB (10.94GB free)| |Process Argv|--crash-reporter-id 7653d454-ecf1-40e5-ad93-80b2e56cc4b5| |Screen Reader|no| |VM|67%| </details><details><summary>Extensions (6)</summary> Extension|Author (truncated)|Version ---|---|--- vscode-eslint|dba|2.1.13 denizenscript|den|0.3.5 prettier-vscode|esb|5.8.0 discord-vscode|icr|4.0.0 python|ms-|2020.11.371526539 jupyter|ms-|2020.11.392013122 </details> <!-- generated by issue reporter -->