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/tests/commands/zip.rs

@@ -51,7 +51,7 @@ fn zips_two_lists() {
     let actual = nu!(
         cwd: ".", pipeline(
         r#"
-            echo [0 2 4 6 8] | zip [1 3 5 7 9] | flatten | into string | str collect '-'
+            echo [0 2 4 6 8] | zip [1 3 5 7 9] | flatten | into string | str join '-'
         "#
     ));