build: Stop compiling gpui's test-support into the story examples (#2799)

## Description

There are some issues that complain about "laggy scrolling" and similar,
I believe this comes from the story crate pulling in `test-support`
which has the downside of `App::flush_effects` synchronously drawing at
the end of every `App::update`.
I've only tested this on Linux, maybe it doesn't affect macOS as much?

This moves the dependency into a feature on crates/story, and narrows
the default members.

## Videos

### Before


https://github.com/user-attachments/assets/16a8f40e-d602-4355-8728-2b06fab7bb37


### After


https://github.com/user-attachments/assets/b46f127a-41ea-46b9-856b-2d65780fecc6


## How to Test

Tested by running any example and scrolling. Also verified by grepping
the symbol table for `TestAppContext`,
```sh
nm target/release/examples/markdown | grep TestAppContext
0000000004be36c0 t _RINvXs8_NtCsjjYRrL0dF1h_4gpui3appNtB6_3AppNtB8_10AppContext13update_windowuNCNCINvMsr_NtB8_6windowNtB1m_6Window5deferNCNCINvMs7_B6_Bv_11open_windowNtNtB8_7element5EmptyNCNvMs_NtB6_12test_contextNtB2N_14TestAppContext16add_empty_window0E00E00EB8_
```
None after this change.

## Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [x] Passed `cargo run` for story tests related to the changes.
- [ ] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific)