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/reject.rs

@@ -13,7 +13,7 @@ fn regular_columns() {
             ]
             | reject type first_name
             | columns
-            | str collect ", "
+            | str join ", "
         "#
     ));
 
@@ -56,7 +56,7 @@ fn complex_nested_columns() {
             | reject nu."0xATYKARNU" nu.committers
             | get nu
             | columns
-            | str collect ", "
+            | str join ", "
         "#,
     ));
 
@@ -75,7 +75,7 @@ fn ignores_duplicate_columns_rejected() {
             ]
             | reject "first name" "first name"
             | columns
-            | str collect ", "
+            | str join ", "
         "#
     ));