Embed default `--web-dir`

#### Is your feature request related to a problem? Please describe

I'm always frustrated when ... 😄
I cannot put `waved` binary somewhere in my `$PATH` and use it from working directory.

```
/MY/PWD $ waved --flags

panic: failed reading default index.html page: open /MY/PWD/www/index.html: no such file or directory

goroutine 1 [running]:
github.com/h2oai/wave.Run(0x8c7518, 0x6, 0x8c7e80, 0xe, 0x84fd2c, 0x6, 0x84f448, 0x1, 0xc000156120, 0x28, ...)
/home/runner/work/wave/wave/server.go:124 +0x2145
main.main()
/home/runner/work/wave/wave/cmd/wave/main.go:221 +0x139
```

Sure, I can specify `--web-dir` on each call or create an alias. But in that case I'd need to update the www data with new versions of `waved`.

`0.20.0+` wave includes `waved` with the Python package but more advanced flags are not exposed (e.g. all `--oidc-` flags) so running `waved` separately is necessary for particular cases.

#### Describe the solution you'd like

Embed www data into the binary and load them when the `--www-dir` is not explicitly specified.

Start by reading wave/server.go around the default web-directory handling and cmd/wave/main.go where command-line flags are configured. Trace how an explicitly supplied --web-dir is distinguished from the default, then verify that the binary can use embedded www data when no directory is supplied while preserving the explicit override.

Written by the indexing model from the issue text.