Add ast-grep to tool comparison

[HerringtonDarkholme]

Hi, ast-grep is a code tool that performs the structural search and replace.

Some features that make it unique

• it searches abstract syntax trees instead of text
• it respects VCS ignore-file like .gitignore
• it utilizes multi-cores to speed up searching
• it supports replace file content based on matches

I would appreciate a lot if it can be added to beyondgrep's website. Thank you.