Grafana/ui: Unify flex shorthand props

[Clarity-89]

Currently, the API between the Stack and Box components is inconsistent: Stack supports a flexGrow prop, while Box has a grow prop, which is then used to set the value of the flex shorthand property. Beyond this inconsistency, assigning only the grow value as the complete value of flex sets the shrink and basis implicitly. While this approach works fine in most cases, it creates some edge cases that are not immediately obvious. For instance, by setting flex: 1 on the element, flex-basis is set to 0, and this value will overwrite any width value on the component should we add those in the future (flex-basis, unless set to auto, has precedence over width). For this reason, it is better to be explicit when setting flex values.

What is this feature?

• Unify the flex shorthand props across Box and Stack components.
• Enable passing individual flex props (grow, shrink and basis) as well as the shorthand flex prop.
• Move common types to a separate file.
• Make the Stack story not to be internal-only.

Why do we need this feature?

Consistent API across the layout components.

Which issue(s) does this PR fix?:

Part of ##76516

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[JoaoSilvaGrafana]

Lgtm!