Add keepMounted flag to Modal/Dialog

**Is your feature request related to a problem? Please describe.**
Yes. It would be nice to persist the content sent to a modal/dialog even after it is closed to improve rendering performance.

**Describe the solution you'd like**
The default behavior of mounting the content when the modal/dialog is visible and unmounting it on close animation is great. However, I would like a way to keep the content mounted regardless if it is open. Two options could exist:
1. `after-first` - After opening for the first time, mount the content and keep keep it mounted for subsequent opens
2. `always` - Eagerly mount the content right away and again keep it mounted

Proposing new flag `mountingBehavior` on modal/dialog of type: `'open-only' | 'after-first' | 'always'` defaulted to `'open-only'`

**Describe alternatives you've considered**
I'm in the process of creating a custom dialog to accomplish this.

**Additional context**
[MUI](https://mui.com/material-ui/api/modal/#modal-prop-keepMounted) has a `keepMounted` flag for this.

Start by locating the Modal and Dialog entry points and their current open-only mounting behavior. Review how the proposed mountingBehavior values—open-only, after-first, and always—should apply to both components. Done means the new default preserves existing behavior and each requested option keeps content mounted as specified.

Written by the indexing model from the issue text.