Fix constants with heredoc values

[Morriar]

As reported in #406, constants values are truncated so parsing this input:

FOO = <<EOF
  bar
EOF

will incorrectly output the following RBI:

FOO = <<EOF

The problem stems from the location Prism returns for an heredoc.

Consider this snippet:

require "prism"

ruby_string = <<~RUBY
  FOO = <<~EOF
    foo
  EOF
RUBY

puts Prism.parse(ruby_string).inspect

Prism will return the following AST:

#<Prism::ParseResult:0x000000011f9556f8 @value=@ ProgramNode (location: (1,0)-(1,12))
├── flags: ∅
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,12))
    ├── flags: ∅
    └── body: (length: 1)
        └── @ ConstantWriteNode (location: (1,0)-(1,12))
            ├── flags: newline
            ├── name: :FOO
            ├── name_loc: (1,0)-(1,3) = "FOO"
            ├── value:
            │   @ StringNode (location: (1,6)-(1,12))
            │   ├── flags: ∅
            │   ├── opening_loc: (1,6)-(1,12) = "<<~EOF"
            │   ├── content_loc: (2,0)-(3,0) = "  foo\n"
            │   ├── closing_loc: (3,0)-(4,0) = "EOF\n"
            │   └── unescaped: "foo\n"
            └── operator_loc: (1,4)-(1,5) = "="

Note how the closing_loc points to the correct location at the end of the EOF marker yet the node location (@ StringNode (location: (1,6)-(1,12))) only includes the opening marker.

Digging a bit in Prism issues, I found ruby/prism#1260 explaining this behavior is actually desirable.

Given this, I think the best way is to adjust the Prism location when we encounter a constant that holds a heredoc value.

I also fixed the printing of multiline constants so we insert a blank line as we do with other multiline definitions.

Closes #406.

[apiology]

Appreciate the extra tests!

[apiology]

Works when run against my codebase.

Agreed that fixing this up at the Prism layer is more elegant!

[paracycle]

I am not sure if these examples are exhaustive enough. For example, a common pattern in the CRuby codebase is to use something like:

Foo = "#{<<~begin}#{<<~end}"
begin
  foo
  foo
end

like in here: https://github.com/ruby/ruby/blob/eafcdc153560fd391dabd60705cb88e3f72d7b47/test/ruby/test_file_exhaustive.rb#L760-L773 and this is the main reason why heredocs aren't necessarily contiguous in their begin-end range, and why the tag that represents the heredoc is conceptually separate from the body of it.

I think we need to test to see how multiple heredocs on the same line will interact with this.

[paracycle]

Do we even need the constant value for the RBI? Can't we just replace it with T.unsafe(nil) and add the relevant type for it and move on?

I am saying this not just for constants that have a heredoc value, but all constants in general.