update indicator style to have visibility:hidden for screen readers

[MichaelWest22]

Description

Updating indicator inline style tag so that it uses visibility:hidden as well as opacity so that screen readers will not announce the invisible indicators to users to improve default accessibility

Note I found the current indicator only does a fade in when it shows and does a instant vanish after the request which has been this way for 2 years since a bug was fixed to prevent the indicators fade out showing briefly on hx-location page loads. I kept the same behavior but just added visibility.

Also I found there was a lot of wasted white space being added in the old code which impacted minification so resolved this and optimized the duplicate class config variable use with const's.

Also updated the hx-indicator documentation to match the new default style and improved the documentation around how to disable and work around inline style with CSP headers.

Corresponding issue:
#3354

Testing

Tested manually to confirm the updated style worked as before for fade in during requests and ran the test suite. Also inspected the impact on the minified and gz size to make sure it is smaller than the old one

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded