Trrack selecting and deselecting intersections

[NateLanza]

Does this PR close any open issues?

Closes #257
Closes #332

Give a longer description of what this PR addresses and why it's needed

Selecting an intersection changes the plot in a meaningful way that should be trracked; this adds a Trrack action for selections and deselections and includes the currently selected row in the plot state (including the JSON export).

Provide pictures/videos of the behavior before and after these changes (optional)

Before:

After:

Have you added or updated relevant tests?

• Yes
• No changes are needed

Have you added or updated relevant documentation?

• Yes
• No changes are needed

Are there any additional TODOs before this PR is ready to go?

TODOs:

• Is adding the 'selected' field to the state export going to break any connected apps? The alttxt generation still works fine and passes its test.

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[JakeWags]

Overall, excellent work! A few questions about replacing the verbose if statements (x.y === null) with x?.y === blah and the conditions when those could fail (if they will).

Major change requested is to clean the trrack-vis e2e test so that it is readable and modifiable.

[JakeWags]

Looks good. I don't see any issues with the merge.