respect externals in vercel adapter

[FredKSchott]

Changes

• I was helping @theobr with puppeteer and ran into this issue.
• I marked "chrome-aws-lambda", "puppeteer-core" as external in my Astro config, but then the vercel adapter blew this away and bundled them all again.
• This is a quick fix. @JuanM04 is there a reason to use esbuild.build() instead of esbuild.transform() since we're only ever transforming a single file from ESM -> CJS?

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[JuanM04]

there a reason to use esbuild.build() instead of esbuild.transform() since we're only ever transforming a single file from ESM -> CJS?

It's transforming and bundling dependencies. In my other PR about this (#3051), it only bundles the dependencies

[FredKSchott]

ah, yea I'm realizing this is more complicated than I thought. Closing this in favor of figuring out #3051 first

[JuanM04]

I was helping TheoBr with puppeteer and ran into this issue.

Hey @FredKSchott, do you have a reproduction? I think I've fixed it (or could be fixed) in #3216