feat: allow npm flags (e.g. --install-links) in default-packages file

[ousamabenyounes]

nvm_get_default_packages now permits npm flags (tokens starting with -) alongside a package on a line, or on their own line, instead of rejecting any line containing whitespace. Lines with 2+ actual packages are still rejected. Fixes #3866.

Test verification (RED → GREEN)

With the fix reverted, the new test fails (RED):

Only one package per line is allowed in `/tmp/tmp.TK9aPs5jOf/default-packages`. Please remove any lines with multiple space-separated values.
6: expected default packages >rimraf my-internal-tool --install-links --foreground-scripts<; got >rimraf<

With the fix applied, the test passes (GREEN):

GREEN attempt 1/2 (exit 0)
6: ok: >rimraf my-internal-tool --install-links --foreground-scripts<
7: ok: >Only one package per line is allowed in `/tmp/tmp.nLf2gNVUfX/default-packages`. Please remove any lines with multiple space-separated values.<

Full local suite

Command: npm run test/fast

+ type setopt
+ type shopt
+ rm -fR v* src alias test/test-xz
+ cd ../..
+ type setopt
+ type shopt
+ rm -Rf v* src alias
+ mkdir src alias
+ cd ../..
+ type setopt
+ type shopt
+ rm -fR v* src alias test/test-xz
+ cd ../..
+ type setopt
+ type shopt
+ rm -Rf v* src alias
+ mkdir src alias
+ cd ../..
+ type setopt
+ type shopt
+ rm -fR v* src alias test/test-xz
+ cd ../..
+ type setopt
+ type shopt
+ rm -Rf v* src alias
+ mkdir src alias
+ cd ../..
+ type setopt
+ type shopt
+ rm -fR v* src alias test/test-xz
+ cd ../..
+ [ -d bak ]
+ mv bak/alias bak/versions .
+ rmdir bak
+ mkdir -p src alias

Done, took 205 seconds.
223 tests passed.
�[31m6 tests failed.
�[mmake: *** [Makefile:43: test-bash] Error 6

Fix #3866

[ljharb]

This PR was premature; the issue does not have "help wanted" and it's not certain it's even a feature I want to have.