Add browser support linting

[varholak-peter]

Description

Adds a way to check whether the functionality is compatible with specified browserlist.

Aproach taken

I used the eslint-plugin-builtin-compat in order to check for functionality not supported by browsers specified in browserlist.
I did not include it in the current lint command as it serves a bit different purpose in my opinion, so I created a separate .eslintrc config for it.
I also added npm run supportedbrowsers to list minimum version of all supported browsers.

Disclaimer

The browser list specified is not a final be all end all that I decided is the minimal version Svelte will support. Instead, it is the minimal config that currently passes the check and can be improved by various polyfills.

Closes #558
Closes #2788

[maxmilton]

One big issue I see with the eslint-plugin-builtin-compat package is it's not maintained. One of its dependencies is mdn-browser-compat-data with a pinned version which is very out of date.

[Conduitry]

Which code is it that this is checking for compatibility with specific browsers?