Extension `README.md` preview markdown codeblock language detection

[RedCMD]

The extension README.md preview page does not ignore arguments after the initial whitespace separated languageId ❌

as seen in Github's markdown
and both VSCode's builtin Markdown extensions
all text after the whitespace separator is ignored ✅
(note: json-textmate should fail)

Github: ✅

```json
{
	"key": "value"
}
```

```json textmate
{
	"key": "value"
}
```

```json-textmate
{
	"key": "value"
}
```

Markdown code: ✅

Markdown preview: ✅

Extension page README.md preview: ❌

you can download https://marketplace.visualstudio.com/items?itemName=RedCMD.lag-syntax-highlighter as a simple test extension

PR: #205329

https://github.github.com/gfm/#example-113

vscode/extensions/markdown-basics/syntaxes/markdown.tmLanguage.json

Line 1029 in d84c946

"begin": "(^|\\G)(\\s*)(`{3,}|~{3,})\\s*(?i:(json|json5|sublime-settings|sublime-menu|sublime-keymap|sublime-mousemap|sublime-theme|sublime-build|sublime-project|sublime-completions)((\\s+|:|,|\\{|\\?)[^`]*)?$)",

vscode/src/vs/workbench/contrib/markdown/browser/markdownDocumentRenderer.ts

Line 218 in d84c946

const languageId = languageService.getLanguageIdByLanguageName(lang);

vscode/src/vs/editor/common/services/languagesRegistry.ts

Lines 343 to 349 in d84c946

	public getLanguageIdByLanguageName(languageName: string): string | null {
	const languageNameLower = languageName.toLowerCase();
	if (!hasOwnProperty.call(this._lowercaseNameMap, languageNameLower)) {
	return null;
	}
	return this._lowercaseNameMap[languageNameLower];
	}