[Feature Request] Add plain text support.

[zhuozhiyongde]

As the title suggests, shiki is being used as the code rendering module for my blog, but when I specify text language, it will not render because it cannot recognize this language. I hope to add support for plain text and further, perhaps render all unknown language formats in a form that can be displayed as plain text.

[nova1751]

Actually, if you don't specify the code block lang option,the shiki will return the original code block format.Hope this can help you.

[antfu]

We treat these languages are plain text:

shiki/packages/core/src/utils.ts

Lines 15 to 22 in c791538

	/**
	* Check if the language is plaintext that is ignored by Shiki.
	*
	* Hard-coded plain text languages: `plaintext`, `txt`, `text`, `plain`.
	*/
	export function isPlainLang(lang: string | null | undefined) {
	return !lang || ['plaintext', 'txt', 'text', 'plain'].includes(lang)
	}