Fix markdown light and dark mode when using high contrast themes (microsoft#203690)

Author: luctowers

extensions/github/markdown.css

@@ -4,6 +4,8 @@
  *--------------------------------------------------------------------------------------------*/
 
 .vscode-dark img[src$=\#gh-light-mode-only],
-.vscode-light img[src$=\#gh-dark-mode-only] {
+.vscode-light img[src$=\#gh-dark-mode-only],
+.vscode-high-contrast img[src$=\#gh-light-mode-only],
+.vscode-high-contrast-light img[src$=\#gh-dark-mode-only] {
 	display: none;
 }