Fixer for missing package import that does npm install

[nguerrera]

Combining this with #710, you could do a whole end-to-end experience in the VS Code UI without dropping to the command line. Extensions -> Install cadl, then type, for example, `import "@cadl-lang/rest", then Ctrl+'.' -> select the npm install fix.

[nguerrera]

One problem with the fixer is that after you install a lib then you would get a mismatched compiler until you restart. We would also need to do something about that, which I'll take as tracked by #657

[timotheeguerin]

One issue with this is that users might not be using npm to manage dependencies and if we try to inject with npm it might break their environment