Add `spm update [package]` command

[DecampsRenan]

Add support for updating dependencies. Each package manager has a slightly different command:

spm (proposed)	npm	yarn	pnpm
spm update	npm update	yarn upgrade	pnpm update
spm update foo	npm update foo	yarn upgrade foo	pnpm update foo

Expected behavior

# Update all dependencies
spm update

# Update a specific package
spm update react