`ensure_formula_installed!` requires the `Formula` class to be loaded
before being called to work properly.
Let's guarantee that instead by implementing it as an instance method of
the `Formula` class.
See discussion at #20358.
Fixes
❯ HOMEBREW_BAT=1 brew cat xz
Error: uninitialized constant Kernel::Formula
Warning: Removed Sorbet lines from backtrace!
Rerun with `--verbose` to see the original backtrace
/opt/homebrew/Library/Homebrew/extend/kernel.rb:445:in 'block in <module:Kernel>'
/opt/homebrew/Library/Homebrew/dev-cmd/cat.rb:33:in 'block in Homebrew::DevCmd::Cat#run'
/opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.4.5/lib/ruby/3.4.0/fileutils.rb:241:in 'Dir.chdir'
/opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.4.5/lib/ruby/3.4.0/fileutils.rb:241:in 'FileUtils#cd'
/opt/homebrew/Library/Homebrew/dev-cmd/cat.rb:29:in 'Homebrew::DevCmd::Cat#run'
/opt/homebrew/Library/Homebrew/brew.rb:113:in '<main>'
Please report this issue:
https://docs.brew.sh/Troubleshooting
"cat" is short for "concatenate" (and not "dump to stdout"), so it seems
a little silly for `brew cat` to not be able to concatenate
formulae/casks.
Let's fix that.
