fix(ios): configure SQLite thread safety

[chrispader]

iOS builds currently compile bundled SQLite with SQLITE_THREADSAFE=0, which removes its mutex code even though Nitro runs asynchronous operations on multiple native threads. This makes concurrent SQLite access unsafe, as reported in #312. This PR restores serialized mode by default and lets applications configure it in their package.json.

The nitroSQLite.threadSafe setting accepts true or false and defaults to true. NITRO_SQLITE_THREADSAFE can override it for a Pod installation using true, false, 1, or 0, which is useful in CI. The podspec retains the existing SQLite performance flags, while the documentation explains that this setting has no compile-time effect when NITRO_SQLITE_USE_PHONE_VERSION=1 selects Apple's system SQLite library.

Samuel Scheit introduced the configurable pod setting in PR #298. This PR extracts that work, defaults to 1, and credits him as co-author. It does not include the macOS or CI-platform changes from that PR.

The podspec regression evaluates the package setting, environment override, defaults, invalid values, and system-library configuration. It also compiles bundled SQLite in both supported modes and checks sqlite3_threadsafe().

This fixes the compile-time default. Native connection lifetime and transaction scheduling need separate fixes; SQLite's mutexes do not provide those guarantees.