Allow for customizing the element used to render title of GlAlert

Issue

The title of a GlAlert is hard-coded as an H2 element, but this causes accessibility issues when it is used in a nested position after a non-H1 header.

Proposal

Add a prop to allow for customizing the tag name for the title (would also need to allow for passing in classes for the title OR strengthen the style of the title so that it does not inherit any styles from the semantic h2)

We can default the prop to a semantic H2 and allow for heading level and styling (class) overrides to support nested use cases. I feel this gives the flexibility you're looking for and keeps the changes to the GlAlert API additive and non-breaking.

I keep going back to this article Managing Heading Levels In Design Systems by Heydon Pickering about heading level components and being able to modify them with a level attribute.