This commit adds basic xtask support for building the libsql C library.
This tool allows us to use cargo and rust as the build system. The xtask
system is based off of https://github.com/matklad/cargo-xtask.
This tool provides two commands so far:
- `cargo xtask build` that will build `libsql-sqlite3`.
- `cargo xtask sim-tests` that will run the simulation test suite for
`libsql-server.