Run a V file as a script, without giving up the language.

Usage:
  v [build flags] script.vsh [arguments...]  run the script
  v build script.vsh                         compile it to an executable,
                                             without running it
  v -skip-running script.vsh                 the same thing

A file ending in `.vsh` is parsed in script mode. It is still V, compiled the
same way, and it can use every module V has; two rules are what make it read
like a shell script.

`import os` is added for you when the script does not import `os` itself, so
the usual `os.` prefix is optional:

  mkdir_all('build') or { panic(err) }
  cp('data.txt', 'build/data.txt') or { panic(err) }
  println(args)

The public functions and constants of `os` (e.g. `args`) can be used without
the `os.` prefix; `os` types still need it (`os.File`). A function, local or
parameter of the same name in the script shadows the `os` one. Neither rule
applies to a `.v` file.

Arguments after the script name are passed to the script, not to V.

`v script.vsh` keeps the executable beside the script, named after it
(`script`, or `script.exe` on Windows), and reuses it like `v crun`: it is
rebuilt when the script or the build options change. `v run script.vsh`
rebuilds it on every run.

On Unix-like systems, a script that starts with `#!/usr/bin/env -S v` can be
run directly, once it is made executable with `chmod +x script.vsh`.

A script with no extension runs too, when it starts with a shebang:

  #!/usr/bin/env -S v -raw-vsh-tmp-prefix tmp

`tmp` is the prefix of the built executable, kept as `tmp.<filename>` beside
the script, and that name is overwritten if it already exists. Add `run`
after the prefix to rebuild every time instead of caching.

On systems whose `/usr/bin/env` has no `-S` option, a `.vsh` script can start
with `#!/usr/bin/env vrun` instead (see cmd/tools/vrun); this does not work
for scripts without the `.vsh` extension.

See also `v help build`, `v help crun`, `v help ast` for the AST of a script,
and "Cross-platform shell scripts in V" in doc/docs.md.
