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)

tests/shell/pipeline/commands/internal.rs

@@ -64,7 +64,7 @@ fn subexpression_properly_redirects() {
     let actual = nu!(
         cwd: ".",
         r#"
-            echo (nu --testbin cococo "hello") | str collect
+            echo (nu --testbin cococo "hello") | str join
         "#
     );
 
@@ -215,7 +215,7 @@ fn run_custom_subcommand() {
     let actual = nu!(
         cwd: ".",
         r#"
-        def "str double" [x] { echo $x $x | str collect }; str double bob
+        def "str double" [x] { echo $x $x | str join }; str double bob
         "#
     );
 
@@ -306,7 +306,7 @@ fn run_custom_command_with_rest_other_name() {
                 greeting:string,
                 ...names:string # All of the names
                 ] {
-                    echo $"($greeting), ($names | sort-by | str collect)"
+                    echo $"($greeting), ($names | sort-by | str join)"
                 }
             say-hello Salutations E D C A B
         "#