Chore: Clean up some @grafana/ui SCSS

[ashharrison90]

What is this feature?

• moves Drawer and Slider styles to be imported in the component instead of in separate SCSS files
  • this way webpack/rollup just handles everything
• moves Cascader styles into it's own styles.ts file which is then imported and applied in both Cascader and ButtonCascader
  • here i replaced some old SCSS variables with new theme variables. should be nothing controversial, everything that was picked looks roughly the same and is semantically correct so 🥳

Why do we need this feature?

• clean up old SCSS 🧹

Who is this feature for?

• grafana devs

Which issue(s) does this PR fix?:

For #64129

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[grafana-pr-automation]

⚠️   Possible breaking changes (md version)   ⚠️

grafana-ui

Removals

Cascader.flattenOptions
_{/home/runner/work/grafana/grafana/base/grafana-ui/dist/index.d.ts}

flattenOptions: (options: CascaderOption[], optionPath?: CascaderOption[]) => SelectableValue[];

Cascader.getSearchableOptions
_{/home/runner/work/grafana/grafana/base/grafana-ui/dist/index.d.ts}

getSearchableOptions: micro_memoize.Memoized<(options: CascaderOption[]) => SelectableValue[]>;

Cascader.setInitialValue
_{/home/runner/work/grafana/grafana/base/grafana-ui/dist/index.d.ts}

setInitialValue(searchableOptions: Array>, initValue?: string): {
        rcValue: string[];
        activeLabel: any;
    };

Cascader.onChange
_{/home/runner/work/grafana/grafana/base/grafana-ui/dist/index.d.ts}

onChange: (value: string[], selectedOptions: CascaderOption[]) => void;

Cascader.onSelect
_{/home/runner/work/grafana/grafana/base/grafana-ui/dist/index.d.ts}

onSelect: (obj: SelectableValue) => void;

Cascader.onCreateOption
_{/home/runner/work/grafana/grafana/base/grafana-ui/dist/index.d.ts}

onCreateOption: (value: string) => void;

Cascader.onBlur
_{/home/runner/work/grafana/grafana/base/grafana-ui/dist/index.d.ts}

onBlur: () => void;

Cascader.onBlurCascade
_{/home/runner/work/grafana/grafana/base/grafana-ui/dist/index.d.ts}

onBlurCascade: () => void;

Cascader.onInputKeyDown
_{/home/runner/work/grafana/grafana/base/grafana-ui/dist/index.d.ts}

onInputKeyDown: (e: React__default.KeyboardEvent) => void;

Cascader.onSelectInputChange
_{/home/runner/work/grafana/grafana/base/grafana-ui/dist/index.d.ts}

onSelectInputChange: (value: string) => void;

Cascader.render
_{/home/runner/work/grafana/grafana/base/grafana-ui/dist/index.d.ts}

render(): React__default.JSX.Element;

Cascader.defaultProps
_{/home/runner/work/grafana/grafana/base/grafana-ui/dist/index.d.ts}

static defaultProps: {
        changeOnSelect: boolean;
    };

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