fix: lazy-load SqliteProvider to avoid node:sqlite ExperimentalWarning

[robertsLando]

Summary

• Make SqliteProvider a lazy getter on module.exports so node:sqlite is only required when a consumer actually reads vfs.SqliteProvider.
• Eliminates the process-wide ExperimentalWarning: SQLite is an experimental feature that was emitted on every require('@platformatic/vfs'), even for consumers that only use MemoryProvider / RealFSProvider.

Why

index.js eagerly required ./lib/providers/sqlite.js, which in turn imports node:sqlite at the top of the module. node:sqlite emits an ExperimentalWarning the first time it is loaded in a process. That meant:

• Any app using @platformatic/vfs printed the warning on startup, regardless of whether SQLite was used.
• The noise was especially visible in SEA / pkg binaries where @platformatic/vfs is loaded unconditionally during bootstrap.

The alternative fixes were weaker:

• Suppressing the warning at the consumer layer (e.g. in pkg's sea-bootstrap.js) hides it but keeps the eager load — users still pay the node:sqlite startup cost, and non-SEA consumers still see the warning.
• Splitting into a subpath export (@platformatic/vfs/sqlite) is cleaner long-term but a breaking API change.

The lazy getter is zero API break: the export shape (typeof vfs.SqliteProvider === 'function', enumerable on the object) is preserved, and the value is memoized on first access.

Test plan

• node --test test/sqlite.test.js — all 90 sqlite tests pass.
• Manual repro:
  • require('./index.js') without touching SqliteProvider → no ExperimentalWarning.
  • Reading vfs.SqliteProvider → warning fires exactly once, as expected.
• eslint index.js clean.