Add multiple CSpice build options

[gkjohnson]

It may be prudent to provide multiple versions of cspice.js for different use cases. For example if we can manage to lower the required statically allocated memory for a CSpice module instance we could provide a "thin" version of the source while defaulting to the full 100MB static memory variant. Some applications may prefer loading the ASM.js vs the WASM build depending on which browsers are being targeted.

The different CSpice variants could be guarded by global define (or function argument?) conditions that could dynamically import the appropriate version. If a static global define is used then tree shaking could conditionally remove it.

NOTES

• Replacing instance of 00000] with 0000] brings the initial memory requirement down to ~78MB
• Running a replace again of 0000] to 000] further decreases the requirement down to ~52MB
• The above work in the existing demo file.