[Button] Variant / Tone checks are overly complex

[kyledurand]

Button has variant and tone checks that look like this:

variant === 'primary' && styles.primary,
variant === 'plain' && styles.plain,
variant === 'tertiary' && styles.primary,
variant === 'tertiary' && styles.tertiary,
variant === 'monochromePlain' && styles.monochrome,
variant === 'monochromePlain' && styles.plain,
tone === 'critical' && styles.critical,
tone === 'success' && styles.success,

There are multiple checks for variants that apply different classes leading to code that's harder to maintain. We should be able to simplify this by using the variationName helper and cleaning up some css