Gitpod setup improvements

[shaal]

Changes

Various Gitpod setup improvements:

• Install the latest pnpm in workspace
• Add Astro-build VSCode extension (from open-vsx)
• Run pnpm run build during prebuild (user doesn't have to wait)
• Automatically open preview browser when port 3000 is active
• Run starter demo when workspace starts

Testing

1. Open this PR in Gitpod - https://gitpod.io/#https://github.com/withastro/astro/pull/2789
2. pnpm run build should be ready by the time the workspace open (ran during prebuild)
3. Confirm you can see Astro's starter demo
4. Open .astro file, and confirm syntax highlighting works

• Open blog example from this PR in Gitpod -
  https://gitpod.io/#ASTRO_NEW=blog/https://github.com/withastro/astro/pull/2789
• Open starter example from this PR in Gitpod -
  https://gitpod.io/#ASTRO_NEW=starter/https://github.com/withastro/astro/pull/2789

Docs

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[sarah11918]

Open blog example from this PR in Gitpod -
https://gitpod.io/#ASTRO_NEW=blog/https://github.com/withastro/astro/pull/2789

Works on my machine!

Would be nice if we could also configure /src/pages/index.astro to be open (I don't see in Gitpod docs that we can open a particular file, so I don't know if this is possible) only because this opens the entire astro repository. (See screenshot) So, we would need to include instructions re: navigating down through examples>blog to find the relevant files to work on.

And, here's the syntax highlighting!

[shaal]

@sarah11918 I configured /src/pages/index.astro to open, you can test it with the blog and starter links in the description if this PR

[natemoo-re]

This is excellent! Thank you so much!

[sarah11918]

Yup, this is now working for me, opening to a specific page! This is now my screen on start up, after clicking the link to open the basic starter template: