feat: add a couple of build commands

[henryiii]

Adding a couple of potentially useful build commands:

python -m scikit_build_core.build requires
python -m scikit_build_core.build metadata

"requires" prints out the full build requirements. "metadata" reads the project table, runs dynamic metadata, and then prints the new project table in json form; it may need the requirements installed (which is why I added both commands). Maybe it needs another name, actually, I could see someone wanting the METADATA contents, perhaps.

Like the other commands, these are somewhat experimental and subject to change.

Warning/error messages now print to stderr.