Try to show help pages for external commands w/ help command (#9025)

# Description
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Makes in so if you run `std help <external>` it will run `man <command>`
to get help pages. This command is configurable w/ the `$env.NU_HELPER`
var.

This will close #8032

Examples:
`std help rg` will display the ripgrep help pages

Todo:
- [x] Make flags and fallback configurable
- [x] Improve the warning that it is external
- [ ] Implement `--find` for external commands

# User-Facing Changes
Users will now be able to run `std help` on external commands

# Tests + Formatting
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crates/nu-std/lib/help.nu

@@ -653,7 +653,7 @@ def show-command [command: record] {
     print ""
 }
 
-# Show help on nushell commands.
+# Show help on commands.
 export def "help commands" [
     ...command: string@"nu-complete list-commands"  # the name of command to get help on
     --find (-f): string  # string to find in command names and usage
@@ -663,6 +663,7 @@ export def "help commands" [
     let command = ($command | str join " ")
 
     if not ($find | is-empty) {
+        # TODO: impl find for external commands
         let found_commands = ($commands | find $find --columns [name usage search_terms])
 
         if ($found_commands | length) == 1 {
@@ -671,13 +672,19 @@ export def "help commands" [
             $found_commands | select name category usage signatures search_terms
         }
     } else if not ($command | is-empty) {
-        let found_command = ($commands | where name == $command)
+        let found_commands = ($commands | where name == $command)
 
-        if ($found_command | is-empty) {
+        if not ($found_commands | is-empty) {
+            show-command ($found_commands | get 0)
+        } else {
+            try {
+                print $"(ansi default_italic)Help pages from external command ($command | pretty-cmd):(ansi reset)"
+                 ^($env.NU_HELPER? | default "man") $command
+            } catch {
                 command-not-found-error (metadata $command | get span)
             }
+        }
 
-        show-command ($found_command | get 0)
     } else {
         $commands | select name category usage signatures search_terms
     }