feat: small command line helpers for quick tests (#4272)

* feat: small command line helpers for quick tests

Add --cflags, --ldflags, --embed, and --file to the pybind11 command
line tool, based on python-config. This makes quick one-off compiles
easy:

    c++ $(python3 -m pybind11 --file=example.cpp)

Assisted-by: ClaudeCode:claude-fable-5

* refactor: quote paths, clarify CLI output logic, strengthen tests

- Quote include and library dirs in get_cflags/get_ldflags so paths with
  spaces work, sharing one quote helper in commands.py
- Restructure the --file/--cflags/--ldflags printing into a single print
- Note the Unix-compiler orientation in help text and docs
- Test -L presence for --embed and quoting of paths with spaces

Assisted-by: ClaudeCode:claude-fable-5

* refactor: simplify flag helpers and keep import pybind11 light

- Replace _get_config_var(name, fmt, quote) with a plain _config(name)
  string helper; formatting and quoting happen at the call sites
- Defer sysconfig/shlex imports so import pybind11 does not pay for the
  CLI (~1ms -> ~0.08ms for pybind11.commands)
- Fetch EXT_SUFFIX once in main()
- Import commands normally in the quoting test instead of importlib
  file loading

Assisted-by: ClaudeCode:claude-fable-5

* fix: shared-library flags on all Unix platforms

The --file/--ldflags output only added -shared and -fPIC on Linux, so
other Unix platforms (FreeBSD, Solaris, AIX) linked an executable and
failed on the missing main. Gate on os.name instead.

Assisted-by: ClaudeCode:claude-opus-5