chore: improve v2 types for plot models

[kswenson]

There are no runtime changes associated with this PR, as the changes are primarily restricted to TypeScript types and comments:

• clarify/eliminate some v2 import/export TODO comments
• stricter typing of v2 plot types to assist future implementation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.77%. Comparing base (353d70d) to head (b253710).
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1803      +/-   ##
==========================================
- Coverage   85.84%   85.77%   -0.08%     
==========================================
  Files         636      636              
  Lines       32713    32718       +5     
  Branches     8561     8566       +5     
==========================================
- Hits        28082    28063      -19     
- Misses       4475     4499      +24     
  Partials      156      156              

Flag	Coverage Δ
cypress	73.22% <90.90%> (-0.10%)	⬇️
jest	55.18% <100.00%> (+<0.01%)	⬆️

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[cypress]

codap-v3    Run #6283

Run Properties:   Passed #6283  •  b253710478: chore: improve v2 types for plot models

Project	codap-v3
Branch Review	188616847-v2-plot-types
Run status	Passed #6283
Run duration	06m 17s
Commit	b253710478: chore: improve v2 types for plot models
Committer	Kirk Swenson
View all properties for this run ↗︎

---

Test results
Failures	0
Flaky	1
Pending	53
Skipped	0
Passing	242
View all changes introduced in this branch ↗︎

[bfinzer]

👍🏻LGTM