feat(material/core): add an m2-theme API to define system vars

[andrewseguin]

Add a new mat.m2-theme($config) mixin that existing M2 clients can call to define system-level CSS variables.

This lets clients drop the need to pass around the theme config to custom components and instead just rely on the same CSS variables defined at https://material.angular.dev/guide/system-variables

// Existing M2 theme file

html {
  @include mat.m2-theme($theme-config);
}

// Output

html {
    --mat-sys-background: #faf9fd;
    --mat-sys-error: #ba1a1a;
    --mat-sys-error-container: #ffdad6;
    --mat-sys-inverse-on-surface: #f2f0f4;
    --mat-sys-inverse-primary: #abc7ff;
    --mat-sys-inverse-surface: #2f3033;
    --mat-sys-on-background: #1a1b1f;
    --mat-sys-on-error: #ffffff;
    --mat-sys-on-error-container: #93000a;
    --mat-sys-on-primary: #ffffff;
    ...
}

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