fix(rollup-config): deprecate BundlingConfig#namedExports

this commit deprecates the `namedExports` field on the `BundlingConfig`
interface. `@rollup/plugin-commonjs` no longer honors this field as of
rollup/plugins@5d2dcf4.

Fixes: #2523

STENCIL-867

src/declarations/stencil-public-compiler.ts

@@ -1526,7 +1526,12 @@ export interface CopyTask {
   keepDirStructure?: boolean;
 }
 
+// TODO(STENCIL-882): Remove this interface [BREAKING_CHANGE]
 export interface BundlingConfig {
+  /**
+   * @deprecated the `namedExports` field is no longer honored by `@rollup/plugin-commonjs` and is not used by Stencil.
+   * This field can be safely removed from your Stencil configuration file.
+   */
   namedExports?: {
     [key: string]: string[];
   };