feat: Add --recursive flag to enable env-var nesting

[nkrul]

Added a command line --recursive option.
This will reprocess the env file to fill in any ${variables} there may be.
eg:

VAL1=Hello
VAL2=${VAL1} There

will resolve VAL2 to Hello There
It's not order dependent, as long as it's in the file
It's all done in a single pass.

Also made $var and ${var} both work
Also fixed a return type bug (process.kill can return a boolean in my version of node)

This has been tested on a real project with a file:/ dependency include;

[nkrul]

Look like this matches an existing feature request: #97

[nkrul]

Also possibly #122

[GeorgiosP]

awesome to see this supported out of the box 🎉

[nkrul]

@toddbluhm ?

[markovdigital]

@toddbluhm Hello! It would be great if you could merge it. Variable expansion is essential for most complex projects

[jpdvi]

this would be really useful....

[nkrul]

@toddbluhm - back to a project where this would be really useful.

[toddbluhm]

@nkrul you still interested in this? If your willing to refactor this a tad bit into the current master code base which was just updated to latest typescript, node versions, and esm I will get this included in the next v11 release. If not, I can attempt it myself, just don't want to take your credit.