refactor: Improve GL item config types

[bmingles]

• Broke out explicit ItemConfig sub types and converted ItemConfig type into a union them all
• Updated createContentItem to be more explicit in its mapping of config types to returned content items
• Updated references to ItemConfigType with ItemConfig
• Updated Component, RowOrColumn, and Stack classes to allow null parent

resolves #2130

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 46.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 46.62%. Comparing base (8e47216) to head (08466f9). Report is 4 commits behind head on main.

Files	Patch %	Lines
packages/dashboard/src/layout/LayoutUtils.ts	53.84%	6 Missing :warning:
packages/code-studio/src/main/AppMainContainer.tsx	0.00%	1 Missing :warning:
packages/dashboard/src/PanelManager.ts	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2138 +/- ## ========================================== + Coverage 46.59% 46.62% +0.03% ========================================== Files 682 685 +3 Lines 38441 38492 +51 Branches 9578 9800 +222 ========================================== + Hits 17912 17948 +36 + Misses 20519 20492 -27 - Partials 10 52 +42 ``` | [Flag](https://app.codecov.io/gh/deephaven/web-client-ui/pull/2138/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deephaven) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/deephaven/web-client-ui/pull/2138/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deephaven) | `46.62% <46.66%> (+0.03%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deephaven#carryforward-flags-in-the-pull-request-comment) to find out more.

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