🐛 BUG: Shiki missing language registration should not fail markdown processing

[crutchcorn]

What version of astro are you using?

1.0.0-beta.65

Are you using an SSR adapter? If so, which one?

None

What package manager are you using?

npm

What operating system are you using?

Windows

Describe the Bug

When using Shiki, the following Markdown will fail:

```nasm
100001 # addu

With the following error:

error Failed to parse Markdown file "C:\Users\crutchcorn\git\Astro\unicorn-astro\content\blog\how-computers-speak\index.md": No language registration for nasm at getGrammar (C:\Users\crutchcorn\git\Astro\unicorn-astro\node_modules\shiki\dist\index.js:2175:19) at codeToThemedTokens (C:\Users\crutchcorn\git\Astro\unicorn-astro\node_modules\shiki\dist\index.js:2184:30) at Object.codeToHtml (C:\Users\crutchcorn\git\Astro\unicorn-astro\node_modules\shiki\dist\index.js:2201:24) at file:///C:/Users/crutchcorn/git/Astro/unicorn-astro/node_modules/@astrojs/markdown-remark/dist/remark-shiki.js:17:30 at overload (file:///C:/Users/crutchcorn/git/Astro/unicorn-astro/node_modules/unist-util-visit/index.js:50:16) at node (code) (file:///C:/Users/crutchcorn/git/Astro/unicorn-astro/node_modules/unist-util-visit-parents/index.js:104:31) at node (root) (file:///C:/Users/crutchcorn/git/Astro/unicorn-astro/node_modules/unist-util-visit-parents/index.js:121:79) at visitParents (file:///C:/Users/crutchcorn/git/Astro/unicorn-astro/node_modules/unist-util-visit-parents/index.js:61:30) at visit (file:///C:/Users/crutchcorn/git/Astro/unicorn-astro/node_modules/unist-util-visit/index.js:42:7) at file:///C:/Users/crutchcorn/git/Astro/unicorn-astro/node_modules/@astrojs/markdown-remark/dist/remark-shiki.js:16:5 (x2)

As @Princesseuh pointed out on Discord, it seems like this behavior is thrown from Shiki itself

https://github.com/shikijs/shiki/blob/b0cade2f48f3d6e5c0897c81d0dd06c81f5916ab/packages/shiki/src/highlighter.ts#L108

But Shiki's official `remark` plugin seems to catch this error:

https://github.com/shikijs/twoslash/blob/main/packages/remark-shiki-twoslash/src/index.ts#L167-L172

I think this should be the same behavior we seek to implement in Astro. Otherwise, it enables other's content to break a build, which seems to be too harsh of an action

### Link to Minimal Reproducible Example

https://stackblitz.com/edit/github-nxlpqd?file=src%2Fpages%2Fposts%2Findex.md&on=stackblitz

### Participation

- [X] I am willing to submit a pull request for this issue.

[matthewp]

What behavior are you expecting to happen here? I can't tell from the linked code.

[Princesseuh]

What I'd personally expect is the following:

• Fallback to plaintext automatically whenever the language doesn't exist instead of throwing an error and crashing the server
• Show a warning / error in the console, something like "The chosen language {language} doesn't exist, falling back to plaintext"

[crutchcorn]

100% agree with @Princesseuh in her expected behavior. This is what the ""upstream"" remark-shiki-twoslash plugin does.