Fix highlight notifications increasing when total notification is zero

[germain-gg]

Fixes element-hq/element-web#23885

This was originally as an attempt to be more defensive and prevent highlight notification to appear for already read events!
The synthetic read receipts should help

This clears the release blocker and will monitor the tracker

It also fixes an issue where public get timeline(): MatrixEvent[] would always return undefined because the public abstract timeline: MatrixEvent[] declaration in the parent was overriding it.
It created a lot of inconsistencies when trying to determine whether a message has been previously read or not!

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

🐛 Bug Fixes

• Fix highlight notifications increasing when total notification is zero (#2937). Fixes Receiving a mention in an encrypted "Mentions Only" room does not highlight the room red element-hq/element-web#23885.

[t3chguy]

Looks sane

[RiotRobot]

The backport to staging failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-staging staging
# Navigate to the new working tree
cd .worktrees/backport-staging
# Create a new branch
git switch --create backport-2937-to-staging
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 53a45a34dffc500a21af7774c1a9217f5733cf0f
# Push it to GitHub
git push --set-upstream origin backport-2937-to-staging
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-staging

Then, create a pull request where the base branch is staging and the compare/head branch is backport-2937-to-staging.