[rust] add node_ext module with full_name methods for constant nodes

[sei40kr]

Summary

• Port Ruby's node_ext.rb functionality to Rust bindings
• Add full_name and full_name_parts methods for ConstantReadNode, ConstantWriteNode, ConstantTargetNode, ConstantPathNode, and ConstantPathTargetNode
• Add ConstantPathError enum for error handling

Test Coverage

Rust bindings include 11 unit tests compared to Ruby's 8 tests. Additional coverage includes:

• ConstantWriteNode (not tested in Ruby)
• ConstantTargetNode (not tested in Ruby)
• MissingNodes error case (not tested in Ruby)

Public API Changes (cargo-public-api)

Added items to the public API
=============================
+pub enum ruby_prism::ConstantPathError
+pub ruby_prism::ConstantPathError::DynamicParts
+pub ruby_prism::ConstantPathError::MissingNodes
+impl core::clone::Clone for ruby_prism::ConstantPathError
+impl core::cmp::Eq for ruby_prism::ConstantPathError
+impl core::cmp::PartialEq for ruby_prism::ConstantPathError
+impl core::error::Error for ruby_prism::ConstantPathError
+impl core::fmt::Debug for ruby_prism::ConstantPathError
+impl core::fmt::Display for ruby_prism::ConstantPathError
+impl<'pr> ruby_prism::ConstantPathNode<'pr>
+pub fn ruby_prism::ConstantPathNode<'pr>::full_name(&self) -> Result
+pub fn ruby_prism::ConstantPathNode<'pr>::full_name_parts(&self) -> Result>, ConstantPathError>
+impl<'pr> ruby_prism::ConstantPathTargetNode<'pr>
+pub fn ruby_prism::ConstantPathTargetNode<'pr>::full_name(&self) -> Result
+pub fn ruby_prism::ConstantPathTargetNode<'pr>::full_name_parts(&self) -> Result>, ConstantPathError>
+impl<'pr> ruby_prism::ConstantReadNode<'pr>
+pub fn ruby_prism::ConstantReadNode<'pr>::full_name(&self) -> Cow<'pr, str>
+pub fn ruby_prism::ConstantReadNode<'pr>::full_name_parts(&self) -> Vec>
+impl<'pr> ruby_prism::ConstantTargetNode<'pr>
+pub fn ruby_prism::ConstantTargetNode<'pr>::full_name(&self) -> Cow<'pr, str>
+pub fn ruby_prism::ConstantTargetNode<'pr>::full_name_parts(&self) -> Vec>
+impl<'pr> ruby_prism::ConstantWriteNode<'pr>
+pub fn ruby_prism::ConstantWriteNode<'pr>::full_name(&self) -> Cow<'pr, str>
+pub fn ruby_prism::ConstantWriteNode<'pr>::full_name_parts(&self) -> Vec>

Test plan

• bundle exec rake cargo:test passes
• bundle exec rake cargo:lint passes
• Doctests pass

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