fix: upgrade deps (thlorenz#225)

This updates all dependencies to their latest version.

Specifically, this resolves an issue with a vulnerable version of `[email protected]` in the current dependency tree.
```
$ npm ls trim
[email protected] /workspaces/doctoc
└─┬ @textlint/[email protected]
  └─┬ [email protected]
    └── [email protected] 
```

More details here: GHSA-w5p7-h5w8-2hfq

I had to slightly tweak one test to account for a change in the markdown parser (heading not recognized when part of a previous paragraph).