Optional @finos/perspective-webpack-plugin

[texodus]

Makes @finos/perspective default to export a WASM-inlined build, which inlines the WASM payload as base64 and the WebWorker as an inline string. With this change, by default:

• All @finos/perspective* modules will build without @finos/perspective-webpack-plugin.
• All bundlers (including webpack) will work without special configuration.

But:

• Bundle outputs will be ~500kb larger (due to base64 encoding overhead).
• Initial load will be slower (though engine runtime once loaded will not be affected).
• Debuggers which try to load the raw JS file may go haywire.

So by default, perspective will now be easier to install, but output a slightly worse build. By using @finos/perspective-webpack-plugin, the build will still work as currently, outputting the independent perspective.wasm.worker.js and psp.async.wasm, with no performance impact.

This is intended to address many recent support questions about build errors for new users e.g. #860 #857

On the side, this also makes perspective-test output screenshots from different architectures to separate folders, which makes testing across docker/local puppeteer much more convenient.