Environment-aware help for open and save (#15651)

# Description

This extends the documentation on the commands `open` and `save` can run
under the hood, and explicitly lists those, based on the current user
environment.

Also see [this discord
thread](https://discord.com/channels/601130461678272522/988303282931912704/1364930487092777020)

# User-Facing Changes

Users will be able to see the list of commands that `open` and `save`
can run, and the extensions that each command is run for, in `help open`
and `help save` respectively:

## `help open`

![image](https://github.com/user-attachments/assets/b245d12c-c6ef-4c6d-a9f1-6c5111cb0684)

## `help save`

![image](https://github.com/user-attachments/assets/e92ddb6b-6a1e-40cc-9139-78db8a921d4a)


# Tests + Formatting

All pass except for the ones that don't (and never did pass for me
before).

# After Submitting

No updates needed.

crates/nu-command/src/filesystem/open.rs

@@ -281,6 +281,16 @@ impl Command for Open {
                 example: r#"def "from ndjson" [] { from json -o }; open myfile.ndjson"#,
                 result: None,
             },
+            Example {
+                description: "Show the extensions for which the `open` command will automatically parse",
+                example: r#"scope commands
+    | where name starts-with "from "
+    | insert extension { get name | str replace -r "^from " "" | $"*.($in)" }
+    | select extension name
+    | rename extension command
+"#,
+                result: None,
+            }
         ]
     }
 }

crates/nu-command/src/filesystem/save.rs

@@ -263,6 +263,17 @@ impl Command for Save {
                 example: r#"do -i {} | save foo.txt --stderr bar.txt"#,
                 result: None,
             },
+            Example {
+                description:
+                    "Show the extensions for which the `save` command will automatically serialize",
+                example: r#"scope commands
+    | where name starts-with "to "
+    | insert extension { get name | str replace -r "^to " "" | $"*.($in)" }
+    | select extension name
+    | rename extension command
+"#,
+                result: None,
+            },
         ]
     }