Port find command (#658)

* Add `Find` command

* Complete rustdoc for test `Value` constructors

* Use `Option::unwrap_or` instead of match

* Add `Value::test_filesize` constructor

* Handle searching for terms in `find`

* Fix `find` command signature

* Return multiple elements when `find`ing by predicate

* Do not accept rest parameter with predicate

* Handle `CellPath` in `r#in` and `not_in` for `Find`

* Use `PipelineData::filter`

crates/nu-command/src/default_context.rs

@@ -62,6 +62,7 @@ pub fn create_default_context(cwd: impl AsRef<Path>) -> EngineState {
             Each,
             Empty,
             Every,
+            Find,
             First,
             Flatten,
             Get,