feat: add a warning message if there are no entrypoints

[nahtnam]

Details

Context: https://discord.com/channels/946290204443025438/978320682985349130/1015413639827705906

Code of Conduct

• I agree to follow this project's Code of Conduct
• I checked the current PR for duplication.

Testing

I only tested newtab.tsx, I'm going to assume the underlying initTemplateFile function works as expected

with newtab.tsx:

➜  with-newtab git:(8fa7fff) pnpm run build   

> with-newtab@0.0.0 build plasmo/examples/with-newtab
> plasmo build

🟣 Plasmo v0.52.5
🟠 The browser extension development framework.
🔵 INFO   | Prepare to bundle the extension...
🟢 DONE   | Finished in 2380ms!

after removing newtab.tsx:

➜  with-newtab git:(8fa7fff) pnpm run build

> with-newtab@0.0.0 build plasmo/examples/with-newtab
> plasmo build

🟣 Plasmo v0.52.5
🟠 The browser extension development framework.
🔵 INFO   | Prepare to bundle the extension...
🟠 WARN   | Unable to find any entrypoints. You may end up with an empty extension...
🟢 DONE   | Finished in 669ms!

[louisgv]

LGTM 👍