Update template to Webpack 5 for better experience

[non25]

I find it sad that there's 18k weekly downloads of svelte-loader and these users don't get the best possible svelte experience on Webpack. This template doesn't show a good example of how this could be done either, and it is the most important one, because it is official.

This fixes the situation by updating svelte-loader from master, which has Webpack 5 and HMR support and also updating everything else to the most recent version. I believe starters don't want to use stale deps.

Issues: Fixes #12, Fixes #31, Fixes #34
PRs: Fixes #33, Fixes #24, Fixes #21, Fixes #28

[non25]

I've been thinking about emitCss: false for HMR to work and somehow having bundle.css not to break reloading. The ways to solve this are:

1. Prebuild bundle.css before running devmode. Drawbacks: old css from previous production build interfering with new, need to prebuild stuff
2. Make an empty bundle.css in public before running devmode. Drawbacks: kinda manualish or could introduce wierd stuff in webpack.config.js to make it automatic
3. Move global.css as an import to the main.js. Can't see drawbacks for that, allows HMR to work for global.css, fixes Hot reload, how to watch for global.css too #31.

Currently, components don't output css in devmode, and there's no global imported css, so by default there won't be any bundle.css in devmode in real/virtual fs, so hot reloading will break.

^ That is also because we are using style-loader. Moving global.css to src won't fix #12 by itself.
We will also need to use mini-css-extract-plugin in devmode, but that is preferable nowadays anyway.

[non25]

Also I think #4, #9, #26, #30, #32 are irrelevant now and could be closed.