Use JS extensions for page entrypoints

[matthewmueller]

Right now we have entrypoints that have the following format: bud/view/{route}/_{action}.{ext}, so for bud/view/index.svelte, the entrypoint would look like bud/view/_index.svelte.

This was nice for understanding the mapping, but doesn't play well with tools that use the extension to determine the content-type, since those served svelte files are actually javascript. I ran into this while using https://github.com/tj/staticgen.

Let's change this, perhaps:

• .bud/view/.index.svelte.js: for the entrypoint to view/index.svelte (includes hydration + event sourcing)
• .bud/view/index.svelte.js: the compiled view/index.svelte component that can be imported dynamically

[matthewmueller]

For now, this has been changed to bud/view/_index.svelte.js to fix the immediate issue with staticgen.