Improve wasm32-wasi support

[Starting from 3.41.0 SQLite has wasm32-wasi support out of the
box.][0]

- Set `-DSQLITE_THREADSAFE=0`. Fixes:

```
$ wasmtime target/wasm32-wasi/release/examples/persons.wasm
Error: failed to run main module `target/wasm32-wasi/release/examples/persons.wasm`

Caused by:
    0: failed to instantiate "target/wasm32-wasi/release/examples/persons.wasm"
    1: unknown import: `env::pthread_mutexattr_init` has not been defined
```

- Drop `-DSQLITE_OS_OTHER`. Fixes:

```
$ wasmtime target/wasm32-wasi/release/examples/persons.wasm
Error: failed to run main module `target/wasm32-wasi/release/examples/persons.wasm`

Caused by:
    0: failed to instantiate "target/wasm32-wasi/release/examples/persons.wasm"
    1: unknown import: `env::sqlite3_os_init` has not been defined
```

- [Add wasi specific build flags][1]
- Add basic example
- Also, add instructions how to run it against wasm32-wasi.

Using of file databases is also working, though `--mapdir` arg
should be provided to `wasmtime run`.

[0]: https://wasmlabs.dev/articles/sqlite-wasi-support/
[1]: https://github.com/vmware-labs/webassembly-language-runtimes/blob/main/libs/sqlite/wasi-patches/wlr-build.sh#L11

libsqlite3-sys/build.rs

@@ -239,11 +239,16 @@ mod build_bundled {
         if !win_target() {
             cfg.flag("-DHAVE_LOCALTIME_R");
         }
-        // Target wasm32-wasi can't compile the default VFS
         if env::var("TARGET").map_or(false, |v| v == "wasm32-wasi") {
-            cfg.flag("-DSQLITE_OS_OTHER")
+            cfg.flag("-USQLITE_THREADSAFE")
+                .flag("-DSQLITE_THREADSAFE=0")
                 // https://github.com/rust-lang/rust/issues/74393
-                .flag("-DLONGDOUBLE_TYPE=double");
+                .flag("-DLONGDOUBLE_TYPE=double")
+                .flag("-D_WASI_EMULATED_MMAN")
+                .flag("-D_WASI_EMULATED_GETPID")
+                .flag("-D_WASI_EMULATED_SIGNAL")
+                .flag("-D_WASI_EMULATED_PROCESS_CLOCKS");
+
             if cfg!(feature = "wasm32-wasi-vfs") {
                 cfg.file("sqlite3/wasm32-wasi-vfs.c");
             }