Add support lite build (down to 20MB)

[gkjohnson]

Fix #34
Fix #39

• Add "type" option to init to dictate the compiled version of SPICE to use
• Dynamically load the cspice modules in "init" rather than when loading "Spice.js"
• Update README with updated info and documentation on the "lite" version of the SPICE module
• Move cspice.js to src/cspice/asm_full.js
• Switch the package over to use "type: module" in the package.json
• Added "lite" CSPICE version to src/cspice/asm_lite.js
• Adjust generation script to include support for CSPICE lite
• Add link to internal repo containing modified CSPICE lite code.
• Change package.json version to "alpha4"
• Add "prepublishOnly" script for publishing to npm

TODO

• Add documentation on building the "lite" version of CSPICE (internal only)
• Build "lite" CSPICE version to src/cspice/asm_lite.js
• Adjust the generate script (or make a second one) to write out a second "lite" version of CSPICE
• Switch the generation script over to use ".mjs" extension when building with emscripten to avoid node.js module issues (no __dirname available)

cc @camargo

[camargo]

Thanks so much @gkjohnson!!