Refactor arguments of path subcommands & Add path join subcommand (#3123)

* Refactor path subcommand argument handling

DefaultArguments are no longer passed to each subcommand. Instead, each
subcommand has its own Path<xxx>Arguments. This means that it is no
longer necessary to edit every single path subcommand source file when
changing the arguments struct.

* Add new path join subcommand

Makes it easier to create new paths. It's just a wrapper around Rust's
Path.join().

crates/nu-command/tests/commands/path/mod.rs

@@ -4,6 +4,7 @@ mod exists;
 mod expand;
 mod extension;
 mod filestem;
+mod join;
 mod type_;
 
 use std::path::MAIN_SEPARATOR;