Adds target constructors that make it easy to use Python, Julia, and Rust inside a 'targets' pipeline using 'reticulate', 'JuliaCall', and 'rextendr'. Provides tar_target_py(), tar_target_jl(), and tar_target_rs() (with matching _raw() variants), each mirroring 'targets::tar_target()' and 'targets::tar_target_raw()'. Python and Julia steps run a script via a live interpreter with optional R pre- and post-scripts; Rust steps compile '#[extendr]' functions and call them from an R post-script. Results are returned either as converted R objects or as files written to disk (format = "file"). Dynamic branching, environment/version selection, a 'crew' controller for isolation, and the full set of tar_target_raw() arguments are supported.