Add postinstall script to install the safe npm alias

[charliegerard]

PR description

This PR adds the following functionalities:

• A postinstall script to ask users if they want to install our npm and npx wrapper so they're automatically protected against malicious packages. (It checks first if the aliases already exist and if so, does not prompt the user)
• The commands socket wrapper with the flags --enable and --disable if users want to add/remove the aliases later on. If the user enters socket wrapper --enable, the logic checks if the aliases are already set up and if so, lets the user know, otherwise adds the aliases in the bashrc or zshrc file.
• The commands socket raw-npm and socket raw-npx to temporarily bypass the wrapper.

Flow:

• When the CLI has finished installing, the output looks like this:

• If the user enters "n", the interface just exits.
• If the user enters "y", the logic checks if the file ~/.bashrc or ~/.zshrc exist and if so, the aliases are added to this file and the following output is displayed.

[feross]

@charliegerard This is an excellent idea!

Should we also expose it in a CLI command, in case I deny it but want to do it later? Could be a command like socket wrapper enable and socket wrapper disable?

[charliegerard]

Implemented the feedback to replace echo with fs.appendFile and add the aliases in both the .bashrc and .zshrc files.