Fix for multiple extension entry bug (+ api change) in 'build' subcmd

[shakfu]

In the prior commit there was a bug which prevented multiple extensions from being entered via the build subcmd (it would always take the last one entered).

The below code

build_sp.add_argument("-n", "--ext-name", nargs=True, help="name of extension")
build_sp.add_argument("-p", "--ext-path", nargs=True, help="path of extension")

really should have been

build_sp.add_argument("-n", "--ext-name", nargs='+', help="name of extension")
build_sp.add_argument("-p", "--ext-path", nargs='+', help="path of extension")

But it was found that this would force the user to enter names and files separately:

python -m pydust build \
	--ext-name 'fib._lib' 'fib._hello' \
	--ext-path 'fib/fib.zig' 'fib/hello.zig'

While this is not terrible, this PR proposes the following new api for entering extensions from the commandline instead:

build_sp.add_argument("-e", "--extensions", nargs='+', help="space separated list of extension '<name>=<path>' entries")

which would allow the entry of multiple extensions as follows:

python -m pydust build --extensions 'fib._lib=fib/fib.zig' 'fib._hello=fib/hello.zig'
# or
python -m pydust build -e 'fib._lib=fib/fib.zig' 'fib._hello=fib/hello.zig'