Figure out how to compile zig-sqlite for multithreaded operation in WebAssembly

[chung-leong]

Need to figure out how sqlite decides whether to build for thread safty or not.

[chung-leong]

We need to implement pthread emulation:

• Thread
• Mutex
• Condition
• ReadWriteLock
• Spinlock
• Semaphore
• Key
• Cancellation
• Misc

[chung-leong]

An option to add -DSQLITE_THREADSAFE=1 is also needed in zig-sqlite's build file.