feat: complete environment variables and expand them if possible

fix: avoid E474 "Invalid argument" when canceling search job

chore: update links

docs: remove old links

refactor: use simpler async support check for Vim

feat: add `g:FerretCommandNames`

docs: update HISTORY section

fix: don't E684 if quickfix listing is empty

fix: avoid Vimscript footgun

fix: avoid E42 error when deleting last quickfix entry

fix: rg v13.0.0 hang when passing additional options that take arguments

docs: format leader with backticks

docs: fix a minor typo in the docs

feat: add `<Plug>(FerretBack)`, `<Plug>(FerretBlack)`, `<Plug>(FerretQuack)`

docs: prepare HISTORY section for further development