Fix parent type of some Net::IMAP classes: (sorbet#8392)

- Net::IMAP::Atom: [source](https://github.com/ruby/net-imap/blob/6a520fb1eb3bbeeb1f97fdeed7c425b4f7addaa5/lib/net/imap/command_data.rb#L138)
- Net::IMAP::Literal [source](https://github.com/ruby/net-imap/blob/6a520fb1eb3bbeeb1f97fdeed7c425b4f7addaa5/lib/net/imap/command_data.rb#L150)
- Net::IMAP::MessageSet [source](https://github.com/ruby/net-imap/blob/6a520fb1eb3bbeeb1f97fdeed7c425b4f7addaa5/lib/net/imap/command_data.rb#L157)
- Net::IMAP::QuotedString [source](https://github.com/ruby/net-imap/blob/6a520fb1eb3bbeeb1f97fdeed7c425b4f7addaa5/lib/net/imap/command_data.rb#L144)
- Net::IMAP::RawData [source](https://github.com/ruby/net-imap/blob/6a520fb1eb3bbeeb1f97fdeed7c425b4f7addaa5/lib/net/imap/command_data.rb#L132)

Added a new class for Net::IMAP::CommandData, since that class is
defined
[here](https://github.com/ruby/net-imap/blob/6a520fb1eb3bbeeb1f97fdeed7c425b4f7addaa5/lib/net/imap/command_data.rb#L123-L130)
as `Data.define(:data)`, with two methods, `send_data` and `validate`, I
added all three and initialize in the definition.

The changes were made in this commit, included [in 0.5.2][1]

[1]:https://github.com/ruby/net-imap/releases/tag/v0.5.2

Author: pjambet

rbi/stdlib/net.rbi

@@ -4345,7 +4345,17 @@ class Net::IMAP::Address < Struct
   def self.new(*_); end
 end
 
-class Net::IMAP::Atom
+class Net::IMAP::CommandData < Data
+  def initialize(data); end
+
+  def data; end
+
+  def send_data(imap); end
+
+  def validate(); end
+end
+
+class Net::IMAP::Atom < Net::IMAP::CommandData
   def initialize(data); end
 
   def send_data(imap); end
@@ -5010,7 +5020,7 @@ end
 class Net::IMAP::FlagCountError < Net::IMAP::Error
 end
 
-class Net::IMAP::Literal
+class Net::IMAP::Literal < Net::IMAP::CommandData
   def initialize(data); end
 
   def send_data(imap); end
@@ -5191,7 +5201,7 @@ class Net::IMAP::MailboxQuotaRoot < Struct
   def self.new(*_); end
 end
 
-class Net::IMAP::MessageSet
+class Net::IMAP::MessageSet < Net::IMAP::CommandData
   def initialize(data); end
 
   def send_data(imap); end
@@ -5227,15 +5237,15 @@ class Net::IMAP::PlainAuthenticator
   def process(data); end
 end
 
-class Net::IMAP::QuotedString
+class Net::IMAP::QuotedString < Net::IMAP::CommandData
   def initialize(data); end
 
   def send_data(imap); end
 
   def validate(); end
 end
 
-class Net::IMAP::RawData
+class Net::IMAP::RawData < Net::IMAP::CommandData
   def initialize(data); end
 
   def send_data(imap); end