forked from extern/nushell
* Fixes a panic with defining two commands with the same name caused by declaration not found after predeclaration. * Adds a new error if a custom command is defined more than once in one block. * Add some tests
* Fixes a panic with defining two commands with the same name caused by declaration not found after predeclaration. * Adds a new error if a custom command is defined more than once in one block. * Add some tests