Add executeInput() method to REPL that lexes input, determines if it's a statement or expression based on the first token, and evaluates accordingly using Meta.eval for statements or the new Meta.compileExpression for expressions. Extend Meta.compile_ foreign method with isExpression and printErrors parameters, update wrenCompile signature to support expression compilation mode, and add newline handling after equals in variable definitions.
Replace the simple fgets-based REPL loop with a full-featured interactive Wren REPL module. The new implementation supports cursor movement via arrow keys, Ctrl-A/E for line start/end, Ctrl-C/D for exit/delete, character insertion/deletion, and syntax highlighting of the current input line. The C side now imports the "repl" Wren module and runs the libuv event loop instead of blocking on stdin reads.
