Allow 'start' attribute in rendered markdown (microsoft#174854)

Fixes microsoft#174563

Author: mjbvz

src/vs/base/browser/dom.ts

@@ -1461,7 +1461,7 @@ export const basicMarkupHtmlTags = Object.freeze([
 
 const defaultDomPurifyConfig = Object.freeze<dompurify.Config & { RETURN_TRUSTED_TYPE: true }>({
 	ALLOWED_TAGS: ['a', 'button', 'blockquote', 'code', 'div', 'h1', 'h2', 'h3', 'h4', 'h5', 'h6', 'hr', 'input', 'label', 'li', 'p', 'pre', 'select', 'small', 'span', 'strong', 'textarea', 'ul', 'ol'],
-	ALLOWED_ATTR: ['href', 'data-href', 'data-command', 'target', 'title', 'name', 'src', 'alt', 'class', 'id', 'role', 'tabindex', 'style', 'data-code', 'width', 'height', 'align', 'x-dispatch', 'required', 'checked', 'placeholder', 'type'],
+	ALLOWED_ATTR: ['href', 'data-href', 'data-command', 'target', 'title', 'name', 'src', 'alt', 'class', 'id', 'role', 'tabindex', 'style', 'data-code', 'width', 'height', 'align', 'x-dispatch', 'required', 'checked', 'placeholder', 'type', 'start'],
 	RETURN_DOM: false,
 	RETURN_DOM_FRAGMENT: false,
 	RETURN_TRUSTED_TYPE: true

src/vs/base/browser/markdownRenderer.ts

@@ -388,6 +388,7 @@ export const allowedMarkdownAttr = [
 	'target',
 	'title',
 	'width',
+	'start',
 ];
 
 function getSanitizerOptions(options: { readonly isTrusted?: boolean | MarkdownStringTrustedOptions }): { config: dompurify.Config; allowedSchemes: string[] } {