Anchors starting with a number cause incompability with linting tools

[RaimondB]

• [X ] I confirm that this is an issue rather than a question.

Bug report

An earlier issue #121 that was fixed causes incompatibility with linting tools.
Vuepress will prefix anchorlinks that start with a digit with an underscore.
Widely used tools such as markdownlint and markdownlinkcheck don't do this an therefore break when headings are used that start with a digit and anchors or TOCs are created.

Steps to reproduce

1. Create a vuepress site
2. Add markdownlint or markdownlinkcheck in your CI/CD pipeline
3. Create a heading that starts with a digit
4. Create an internal link to the heading
5. Run the CI/CD pipeline

What is expected?

When CI/CD passes, it should also work in VuePress

What is actually happening?

When CI/CD passes, the link will not work in vuepress. When changed to work in vuepress, the CI/CD will break.

Other relevant information

• Output of npx vuepress info in my VuePress project: