fix(material/core): define token overrides inline

[andrewseguin]

Creates CSS variables for the overrides API inline instead of using a new selector rule for each token

html {
  @include mat.checkbox-overrides(
    (
      disabled-label-color: orange,
      label-text-color: red,
    )
  );
}

Before:

html {
  --mat-checkbox-disabled-label-color: orange;
}
html {
  --mat-checkbox-label-text-color: red;
}

After:

html {
  --mat-checkbox-disabled-label-color: orange;
  --mat-checkbox-label-text-color: red;
}

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