further simplify adding new deps to the root

[isaacs]

The previous revision of the buildIdealTree/reify add option converted
the package.json-esque data structure into instead passing a list of
specs for each location in the package, since the name couldn't be known
100% of the time up front.

However, that still puts too much of the burden of placing deps onto the
caller.

When you run npm install foo, if foo is already a devDependency,
then the updated save spec gets added there, unless specified otherwise.
Since we aren't guaranteed to know the name of the dep until we've
resolved it, how is the caller (ie, the npm CLI) supposed to know where
to put it in the objecet?

At the expense of a slight amount of flexibility, this changes things
further so that the add option mirrors exactly the positional
arguments passed to the npm install command: it's just a list of
specs, that's it.

Two more options were added:

• saveType
• saveBundle

If the saveType is not set, then added deps are added wherever they
already exist, or to dependencies if they're not already present.
Otherwise, it should be set to one of the edge type values:

• 'prod'
• 'dev'
• 'optional'
• 'peer'
• 'peerOptional'

If saveBundle is true, then newly added deps are added to the
bundleDependencies list as well.