Exclude 'aria-readonly' in localSettings selector

[alexisquintero]

What this PR is trying to fix: given https://github.com/glacambre/firenvim/blob/master/tsconfig.json as an example, clicking in the code will trigger firenvim to start an nvim instance. This should not happen, the textarea can't be edited

Tested manually first with the default values

document.querySelectorAll('textarea:not([readonly]), div[role="textbox"]')

and with

document.querySelectorAll('textarea:not([readonly], [aria-readonly]), div[role="textbox"]')

Tested locally by building the extension with DOCKER_BUILDKIT=1 docker build . -t firenvim --output target

Closes #1537

[glacambre]

This is great, thanks a lot for taking the time to do this! I must admit that I was a bit annoyed by this but never really took the time to dig into fix it :)