use github-slugger for anchor

* Use the version that doesn't add count to distinguish identical slugs,
  because I don't know where to initialize a slugger instance.
* Update next to 12, because github-slugger is a pure ESM, which is not
  supported by next < 11. See
  https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c#im-having-problems-with-esm-and-nextjs
* Update react stuff, which is peer dependency of next.
* Remove next-routes, which doesn't seem to be used.

app/pages/index.jsx

@@ -9,6 +9,7 @@ import footnote from 'markdown-it-footnote'
 import markdownItAnchor from 'markdown-it-anchor'
 import markdownItToc from 'markdown-it-toc-done-right'
 import markdownDeflist from 'markdown-it-deflist'
+import GithubSlugger, {slug} from 'github-slugger'
 
 import mk from './katex'
 import chart from './chart'
@@ -205,11 +206,13 @@ export default class PreviewPage extends React.Component {
           permalink: true,
           permalinkBefore: true,
           permalinkSymbol: anchorSymbol,
-          permalinkClass: 'anchor'
+          permalinkClass: 'anchor',
+          slugify: slug,
         })
         .use(markdownItToc, {
           ...DEFAULT_OPTIONS.toc,
-          ...toc
+          ...toc,
+          slugify: x => slug(String(x).trim()),
         })
     }
 

package.json

@@ -17,6 +17,7 @@
     "@chemzqm/neovim": "^5.7.9",
     "@types/markdown-it": "^12.2.3",
     "chart.js": "^2.7.3",
+    "github-slugger": "^2.0.0",
     "highlight.js": "^10.4.1",
     "log4js": "^6.4.0",
     "markdown-it": "^13.0.1",
@@ -28,11 +29,10 @@
     "markdown-it-toc-done-right": "^4.2.0",
     "md-it-meta": "^0.0.2",
     "msgpack-lite": "^0.1.26",
-    "next": "^7.0.2",
-    "next-routes": "^1.4.2",
+    "next": "^12.3.4",
     "plantuml-encoder": "^1.4.0",
-    "react": "^16.5.2",
-    "react-dom": "^16.5.2",
+    "react": "^17.0.2",
+    "react-dom": "^17.0.2",
     "socket.io": "^2.1.1",
     "socket.io-client": "^2.1.1"
   },