Implement smart Enter handler for Elixir

[mkaput]

Add intelligent Enter key handling to automatically indent code, continue multi-line constructs, and provide context-aware line breaks.

Features:

• Auto-indentation after opening delimiters: (, [, {, <<, %{
• Smart line continuation for keyword lists, binary operators, pipe chains
• Context-aware behaviors between delimiters
• Split string literals with proper concatenation
• Respect .editorconfig indentation settings

IntelliJ Extension Points:

• com.intellij.lang.smartEnterProcessor (SmartEnterProcessor)
• com.intellij.codeInsight.editorActions.enterHandler (EnterHandlerDelegate)

See full implementation details in create-issues.sh