Expose veda-ui theme variables for customization in consuming apps

[dzole0311]

Context

The veda-ui components currently rely on a few custom theme variables defined in the _veda-ui-theme-vars file. It may be useful to expose these variables through the veda-ui library so that they can be remapped by the instance. One potential approach is to adopt a similar mechanism to how uswds-theme variables are exposed and used within the consuming applications. This approach would allow the consuming app to remap them like this:

@use 'veda-ui-theme-vars' as * with (
  $veda-uswds-basefont-sans: ...,
  $veda-uswds-fontsize-3xs: ...,
)

Acceptance Criteria

• Given when then
• Given when then

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