[FormLayout] Ensure Item container width is stable (#11976)

### WHY are these changes introduced?

`FormLayout.Item` currently has an `auto` `flex-basis`, which means when
the form initially renders grouped items are equal widths automatically
as we expect, but individual items are allowed to grow when their
content changes instead of maintaining the equal distribution in the
available space. For example, a group of two `TextField` components
starts out with the item containers being equal, but an item can expand
unexpectedly when a `TextField` dynamically has an error message or if
one `TextField` has help text and the other doesn't.

[CodeSandbox
reproduction](https://codesandbox.io/p/sandbox/falling-firefly-xmskmx?file=%2FApp.tsx%3A9%2C1)

### WHAT is this pull request doing?

This PR sets `flex-basis` to `0`, ensuring sibling
`FormLayout.Item`widths are stably equal when grouped.

| Before | After |
|--------|--------|
| ![Screenshot 2024-05-02 at 4 21
04 PM](https://github.com/Shopify/polaris/assets/18447883/c4c00c3a-18b3-4060-91aa-0292dd6d3f92)|![Screenshot
2024-05-02 at 4 21
09 PM](https://github.com/Shopify/polaris/assets/18447883/0e9d35a2-854f-4b98-aaad-774082cb91a5)
|

### How to 🎩

🖥 [Local development
instructions](https://github.com/Shopify/polaris/blob/main/README.md#install-dependencies-and-build-workspaces)
🗒 [General tophatting
guidelines](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md)
📄 [Changelog
guidelines](https://github.com/Shopify/polaris/blob/main/.github/CONTRIBUTING.md#changelog)

🌀
[Spinstance](https://admin.web.form-layout-item-stable-width.chloe-rice.us.spin.dev/store/shop1/customers/new)

### 🎩 checklist

- [ ] Tested a
[snapshot](https://github.com/Shopify/polaris/blob/main/documentation/Releasing.md#-snapshot-releases)
- [x] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [x] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)
- [ ] ~Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)~
- [ ] ~Updated the component's `README.md` with documentation changes~
- [x] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

.changeset/fresh-beds-roll.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+Fixed sibling `FormLayout.Item` widths not remaining equal when wrapped in `FormLayout.Group`

polaris-react/src/components/FormLayout/FormLayout.module.css

@@ -1,7 +1,7 @@
 .Item {
   /* stylelint-disable-next-line -- Polaris component custom properties */
   --pc-form-layout-item-min-size: 220px;
-  flex: 1 1 auto;
+  flex: 1 1 0;
 
   &.grouped {
     /* stylelint-disable-next-line -- Polaris component custom properties */