str collect => str join (#6531)

* Initialize join.rs as a copy of collect.rs

* Evolve StrCollect into StrJoin

* Replace 'str collect' with 'str join' everywhere

git ls-files | lines | par-each { |it| sed -i 's,str collect,str join,g' $it }

* Deprecate 'str collect'

* Revert "Deprecate 'str collect'"

This reverts commit 959d14203e.

* Change `str collect` help message to say that it is deprecated

We cannot remove `str collect` currently (i.e. via
`nu_protocol::ShellError::DeprecatedCommand` since a prominent project
uses the API:

b85542c31c/src/virtualenv/activation/nushell/activate.nu (L43)

crates/nu-command/src/platform/ansi/strip.rs

@@ -40,7 +40,7 @@ impl Command for SubCommand {
     fn examples(&self) -> Vec<Example> {
         vec![Example {
             description: "Strip ANSI escape sequences from a string",
-            example: r#"echo [ (ansi green) (ansi cursor_on) "hello" ] | str collect | ansi strip"#,
+            example: r#"echo [ (ansi green) (ansi cursor_on) "hello" ] | str join | ansi strip"#,
             result: Some(Value::test_string("hello")),
         }]
     }