Better install script

[bhelx]

This improves the extism-js install experience a bit:

• removes the hard coded TAG variable and fetches the latest tag from github api
• tries to install extism-js in a local bin before it asks for sudo and tries a system directory
• is a little more chatty about what it's doing and tells you when it's done and how to verify

[chrisdickinson]

LGTM! Left one comment about the shebang and set -e line out of paranoia