Support the assignment syntax (=> e) in rescue clauses

[coord-e]

TypeProf 264ab6f erroneously reports an undefined method to the following code:

# rescue-assign.rb
def foo(n)
  raise if n != 0
  n.to_s
rescue StandardError => e
  e.message
end

foo(1)

$ bin/typeprof rescue-assign.rb --show-errors
# TypeProf 0.30.1

# rescue-assign.rb
# (6,4)-(6,11):undefined method: nil#message
class Object
  def foo: (Integer) -> String?
end

e in l5 should be recognized as StandardError, not nil. This PR implements the TypeProf support for the assignment syntax => e in rescue clauses, recognizing proper types for the assigned target e.

WIP part

I found a root cause of my concern, and it should be addressed by a separate PR #304.

Details

When there's no exception specified in the clause:

# rescue-assign-no-excs.rb
def foo(n)
  raise
rescue => e
  e
end

foo(1)

I expect foo to be def foo: (Integer) -> untyped because there should be no edge attached to @reference.rhs.ret. However, I get def foo: (Integer) -> nil in the current implementation, and I'm not sure how this can happen...

$ bin/typeprof rescue-assign-no-excs.rb --show-errors
# TypeProf 0.30.1

# rescue-assign-no-excs.rb
class Object
  def foo: (Integer) -> nil
end

[mame]

Awesome! I said at RubyKaigi a (wrong) solution to break a vertex by using "each_type", but yes, InstanceTypeBox is a right solution to fix this issue.