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/flatten.rs

@@ -11,7 +11,7 @@ fn flatten_nested_tables_with_columns() {
                  [[origin, people]; [Nu, ('nuno' | wrap name)]]
             | flatten --all | flatten --all
             | get name
-            | str collect ','
+            | str join ','
         "#
     ));
 
@@ -27,7 +27,7 @@ fn flatten_nested_tables_that_have_many_columns() {
             [[origin, people]; [USA, (echo [[name, meal]; ['Katz', 'nurepa']])]]
             | flatten --all | flatten --all
             | get meal
-            | str collect ','
+            | str join ','
         "#
     ));