Added ESLint with minimal config

[RunDevelopment]

Motivation:
#1796 seems to have become stale because of disagreement over indentation style and the sheer number of changes (look at the number of conflicts).

Description:
This PR adds ESLint with a minimal config. This means that the ESLint config only contains checks for possible errors and best practices. It does not contain style rules.

The idea is to get ESLint now and add style rules later. That way, we can benefit from error checking now and can discuss the style we want to enforce in detail in dedicated PRs. Each dedicated PR will only add/change one rule (or a small number of rules), so we can move quickly and review each rule change individually.

Future PRs:
There are a few PRs I will make as soon as this one got merged:

1. A PR to re-enabling disabled rules with TODOs. A few rules are disabled because fixing them (while possible) isn't easy and because I wanted to keep the diff of this PR as small as possible.
2. A PR to enforces our string quote style.
3. A PR to enforce semicolons.

Changes

• Added ESLint + config
• Added GH actions CI job for linting
• Fixed all linting errors (while trying to keep the diff as minimal as possible)

[github-actions]

JS File Size Changes (gzipped)

A total of 5 files have changed, with a combined diff of -4 B (-0.1%).

file	master	pull	size diff	% diff
components/prism-openqasm.min.js	563 B	562 B	-1 B	-0.2%
components/prism-parser.min.js	664 B	664 B	0 Bytes	0%
components/prism-robotframework.min.js	599 B	598 B	-1 B	-0.2%
plugins/jsonp-highlight/prism-jsonp-highlight.min.js	1.26 KB	1.25 KB	-4 B	-0.3%
plugins/treeview/prism-treeview.min.js	432 B	434 B	+2 B	+0.5%

Generated by 🚫 dangerJS against e5bc929

[LeaVerou]

LGTM! I think that's a good strategy, add minimal ESLint at first, then add rules gradually.

[LeaVerou]

Btw at some point we should rewrite all these var into let and const.

[RunDevelopment]

Btw at some point we should rewrite all these var into let and const.

Guess I'll make a PR for the no-var as well after this :)

[joshgoebel]

Guess I'll make a PR for the no-var as well after this :)

Welcome to the future, Prism.js! Glad to see this happening.

[LeaVerou]

Btw at some point we should rewrite all these var into let and const.

Guess I'll make a PR for the no-var as well after this :)

I am envious of your energy! 😁