feat(cli): support building WASM via podman

Previously, `tree-sitter build-wasm` had the ability to build WASM
by using docker to pull in an image with a complete emscripten toolchain.
This commit adds the ability to use podman to do the same thing.

Using podman requires two notable changes:
1. Using the fully-qualified image name. Docker defaults to prepending
    `docker.io` to the image name, but podman does not.
2. Podman will mount the `/src/` volume as belonging to root unless
  `--userns=keep-id` is passed. I think podman's different
  volume-ownership is related to podman's daemonless execution and
  `--uidmap` functionality, but I'm not 100% sure.

To test, I ran
```sh
script/fetch-fixtures
script/generate-fixtures
script/generate-fixtures-wasm # <- the important one!
```

which worked as well as the docker version.

cli/src/main.rs

@@ -291,9 +291,9 @@ fn run() -> Result<()> {
                 .alias("bw")
                 .about("Compile a parser to WASM")
                 .arg(
-                    Arg::with_name("docker")
-                        .long("docker")
-                        .help("Run emscripten via docker even if it is installed locally"),
+                    Arg::with_name("docker").long("docker").help(
+                        "Run emscripten via docker or podman even if it is installed locally",
+                    ),
                 )
                 .arg(Arg::with_name("path").index(1).multiple(true)),
         )