feat(package): modernize package metadata, typings, and test setup for ESM builds

ts/smartmarkdown.plugins.ts

@@ -1,12 +1,16 @@
+/// <reference path="./smartmarkdown.types.d.ts" />
+
 // pushrocks scope
 import * as smartyaml from '@push.rocks/smartyaml';
 
 export {
   smartyaml
-}
+};
 
 // third party remark
 import { unified } from 'unified';
+import type { Plugin, Transformer } from 'unified';
+import type { Root, RootContent } from 'mdast';
 import remarkGfm from 'remark-gfm';
 import remarkParse from 'remark-parse';
 import remarkFrontmatter from 'remark-frontmatter';
@@ -14,10 +18,19 @@ import remarkHtml from 'remark-html';
 import remarkStringify from 'remark-stringify';
 
 export { unified, remarkGfm, remarkParse, remarkFrontmatter, remarkHtml, remarkStringify };
+export type { Plugin, Transformer, Root, RootContent };
 
 // other third party stuff
 import turndown from 'turndown';
-// @ts-ignore
-import * as turndownPluginGfm from 'turndown-plugin-gfm';
+import * as rawTurndownPluginGfm from 'turndown-plugin-gfm';
+
+type TTurndownPlugin = (serviceArg: InstanceType<typeof turndown>) => void;
+
+const turndownPluginGfm = rawTurndownPluginGfm as {
+  gfm: TTurndownPlugin;
+  strikethrough: TTurndownPlugin;
+  tables: TTurndownPlugin;
+  taskListItems: TTurndownPlugin;
+};
 
 export { turndown, turndownPluginGfm };