Avoid usage of unguarded getRangeAt and add eslint rule

[bor0]

Description

Closes #15606.

This PR addresses #15606. I ran a grep through all getRangeAt calls and fixed all occurrences. It also adds a new eslint rule.

Types of changes

The PR introduces a Bug fix (non-breaking change which fixes an issue), and also adds a rule
enforcement to avoid similar problems in the future

[aduth]

Thanks for this proposal!

Each custom ESLint rule should have its own documentation:

https://github.com/WordPress/gutenberg/tree/master/packages/eslint-plugin/docs/rules

It should also be included in the "Rules" tab of the README.md file:

https://github.com/WordPress/gutenberg/tree/master/packages/eslint-plugin#rules

It's a fairly targeted rule, which makes me wonder if it's the sort of thing where we could generalize the guarding. That said, it's a pretty specific issue with this one function affecting some browsers and not others. I guess we'd really only care to revise it if there were other range functions which are similarly problematic when unguarded.

[bor0]

Thanks @aduth, I updated the PR.

[aduth]

Looks great, thanks! 👍

[aduth]

Some last-minute thoughts / feedback:

• We should include a CHANGELOG entry detailing the rule being added (done in afc18a4).
• I considered (and pushed in 2bf8129) that we name this as "no-" prefixed, rather than "avoid-", since we already have a fair number of theses "avoidance" rules which are consistently named as such. Let me know if you have any thoughts on this.

I was also wondering about the developer experience for someone who encounters this warning. Will it be obvious what they need to do to resolve the issue, when the message is simply "Avoid unguarded getRangeAt" ? I wonder if we could expand the message and/or implement a fixer (depending on difficulty).

It still looks good in its current form, and we could merge it as-is if you're okay with the proposed changes.

[youknowriad]

Can we unblock this. It it seems readyish. Just need a refresh/rebase.

[bor0]

Conflicts resolved, thanks!