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/join.rs

@@ -0,0 +1,45 @@
+use nu_test_support::{nu, pipeline};
+
+use super::join_path_sep;
+
+#[test]
+fn returns_path_joined_with_column_path() {
+    let actual = nu!(
+        cwd: "tests", pipeline(
+        r#"
+            echo [ [name]; [eggs] ]
+            | path join spam.txt name
+            | get name
+        "#
+    ));
+
+    let expected = join_path_sep(&["eggs", "spam.txt"]);
+    assert_eq!(actual.out, expected);
+}
+
+#[test]
+fn appends_slash_when_joined_with_empty_path() {
+    let actual = nu!(
+        cwd: "tests", pipeline(
+        r#"
+            echo "/some/dir"
+            | path join ''
+        "#
+    ));
+
+    let expected = join_path_sep(&["/some/dir", ""]);
+    assert_eq!(actual.out, expected);
+}
+
+#[test]
+fn returns_joined_path_when_joining_empty_path() {
+    let actual = nu!(
+        cwd: "tests", pipeline(
+        r#"
+            echo ""
+            | path join foo.txt
+        "#
+    ));
+
+    assert_eq!(actual.out, "foo.txt");
+}