don't throw Error 'No language registration for xxx'

[gittig11]

When I call getGrammar('mjs') in packages\shiki\src\highlighter.ts, error occurs: No language registration for mjs, which results in a error like 'vitepress build docs' error.

As a result, I pass the default language 'text' to function codeToThemedTokens() when there is no language registration. This will solve the problem.

[gittig11]

PR: #333