Added search terms to if (#13145)

# Description

In this PR, I continue my tradition of trivial but hopefully helpful
`help` tweaks. As mentioned in #13143, I noticed that `help -f else`
oddly didn't return the `if` statement itself. Perhaps not so oddly,
since who the heck is going to go looking for *"else"* in the help?
Well, I did ...

Added *"else"* and *"conditional"* to the search terms for `if`.

I'll work on the meat of #13143 next - That's more substantiative.

# User-Facing Changes

Help only

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`
- 
# After Submitting
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crates/nu-cmd-lang/src/core_commands/if_.rs

@@ -122,6 +122,10 @@ impl Command for If {
         }
     }
 
+    fn search_terms(&self) -> Vec<&str> {
+        vec!["else", "conditional"]
+    }
+
     fn examples(&self) -> Vec<Example> {
         vec![
             Example {