Update package.json for publishing

[wch]

This PR updates package.json for publishing to npmjs.com.

[wch]

I've updated it so that:

• It publishes to @posit/shiny
• It only publishes types

The current build system puts the .d.ts files into srcts/types/, and the built JS files into inst/www/shared. There is also a lot of other stuff in the latter directory.

In the future, I think we'll want to publish the JS and types together in a single package. To do that, I think we should:

• Separate Shiny's JS code from the R code, into a separate repository (or a monorepo with R, Python, and JS subdirs).
• Put all the built JS and .d.ts files into a directory like dist/
• Have the R and Python packages copy the built JS files, or bundle these JS files with their own JS code.

[wch]

Note: It looks like yarn pack does not ignore README-orig.md (so it gets included in the package), but npm pack does ignore the file, which is the behavior that we want.

From what I can find, it's going to require awkward stuff to make yarn pack ignore that file, like moving the temporary file to /tmp. Since we're planning on switching to npm anyway, I suggest we just leave this as it is. We can publish it with npm publish just fine, even if we're using yarn for other steps.