feat(select): expose menu and *-field CSS shadow parts

fixes #4291

PiperOrigin-RevId: 546330782

select/lib/filled-select.ts

@@ -13,4 +13,5 @@ import {Select} from './select.js';
 // tslint:disable-next-line:enforce-comments-on-exported-symbols
 export abstract class FilledSelect extends Select {
   protected readonly fieldTag = literal`md-filled-field`;
+  protected readonly fieldPartName = 'filled-field';
 }

select/lib/outlined-select.ts

@@ -13,4 +13,5 @@ import {Select} from './select.js';
 // tslint:disable-next-line:enforce-comments-on-exported-symbols
 export abstract class OutlinedSelect extends Select {
   protected readonly fieldTag = literal`md-outlined-field`;
+  protected readonly fieldPartName = 'outlined-field';
 }

select/lib/select.ts

@@ -151,6 +151,7 @@ export abstract class Select extends LitElement {
   }
 
   protected abstract readonly fieldTag: StaticValue;
+  protected abstract readonly fieldPartName: string;
 
   /**
    * Used for initializing select when the user sets the `value` directly.
@@ -195,6 +196,7 @@ export abstract class Select extends LitElement {
       <${this.fieldTag}
           aria-haspopup="listbox"
           role="combobox"
+          part=${this.fieldPartName}
           tabindex=${this.disabled ? '-1' : '0'}
           aria-expanded=${this.open ? 'true' : 'false'}
           class="field"
@@ -256,6 +258,7 @@ export abstract class Select extends LitElement {
           listTabIndex="-1"
           type="listbox"
           stay-open-on-focusout
+          part="menu"
           .anchor=${this.field}
           .open=${this.open}
           .quick=${this.quick}