Revise path DSL

[soutaro]

The vendor DSL was broken. This PR introduces new stdlib_path DSL to customize core_root and stdlib_root with keyword argument.

target(:app) do
  stdlib_path(
    core_root: "vendor/rbs/core",              # Load core RBS from vendor/rbs/core/*.rbs
    stdlib_root: nil                           # Load stdlib RBS from default place
  )
end

target(:test) do
  stdlib_path(
    core_root: false,        # Skip loading core RBSs
    stdlib_root: false       # Skip loading stdlib RBSs
  )

  signature "sig"            # You have to write RBS files of core classes in "sig" directory
end