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Switch to INSTALL_SCRIPT for script file

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Add asdf, an extensible tool to manage each of your project runtimes.

asdf is a CLI tool that can manage multiple language runtime versions on
a per-project basis. It is like `gvm`, `nvm`, `rbenv` & `pyenv` (and
more) all in one! Simply install your language's plugin!

Why use asdf?

- single CLI for multiple languages
- consistent commands to manage all your languages
- single global config keeping defaults in one place
- single `.tool-versions` config file per project
- support for existing config files `.node-version`, `.nvmrc`,
  `.ruby-version` for easy migration
- automatically switches runtime versions as you traverse your directories
- simple plugin system to add support for your language of choice
- shell completion available for common shells (Bash, Zsh, Fish, Elvish)