Issue number: **ROU-4815**
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## What is the new behavior?
- Adds an ionic theme style implementation for `ion-button`.
- Adds a new `rectangular` shape to the button.
- Adds two new button sizes: `xsmall` and `xlarge`.
- Adds a new `focus-ring` CSS shadow part for the ionic theme.
## Does this introduce a breaking change?
- [ ] Yes
- [X] No
## Preview
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Co-authored-by: Sean Perkins <13732623+sean-perkins@users.noreply.github.com>
Co-authored-by: Brandy Carney <brandy@ionic.io>
Co-authored-by: Brandy Carney <brandyscarney@users.noreply.github.com>
Co-authored-by: Sean Perkins <sean@ionic.io>
Co-authored-by: Maria Hutt <thetaPC@users.noreply.github.com>
Co-authored-by: João Ferreira <60441552+JoaoFerreira-FrontEnd@users.noreply.github.com>
Co-authored-by: João Ferreira <joao.manuel.ferreira123@gmail.com>
