Create shim executable

[dherman]

Eliminate launchbin and launchscript and replace with a single shim executable:

• Remove the launch{bin, script} binaries
• Create a shim binary
• Installer should install it to ~/.notion/exe/shim
• use https://crates.io/crates/process_path to figure out the executable name
• first try to use the project toolchain (this logic already exists)
• next try to read ~/.notion/tools/user/bins/$name/package.toml
• get the tool path from the toml data
• launch ~/.notion/tools/user/installed/$package/$toolpath (use cmd /c if this is Windows and it's a legacy batch/cmd script)
• finally default to delegating to next thing in PATH

Depends on:

• Package binary case of Toolchain::install #175 Package binary case of Toolchain::install

[charlespierce]

I think the architecture of the code in Volta has moved beyond the specific details of this issue, and we do now have a volta-shim executable that handles the shim behavior.