feat(language-core): add context interface to allow plugins to augment it easily

[KazariEX]

Example:

import type { VueLanguagePlugin } from "@vue/language-core";

const plugin: VueLanguagePlugin = () => {
  return {
    version: 2.1,
    resolveEmbeddedCode(fileName, sfc, embeddedCode) {
      if (!embeddedCode.id.startsWith("script_")) {
        return;
      }
      embeddedCode.content.push(`interface __VLS_Context { $route: ... }`);
    },
  };
};

export default plugin;