Improve #12008 UX, clear scrollback by default on clear (#13821)

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# Description
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Related to #11693. It looks like there is no reason for Nu shell's
`clear` to behave differently than other shells' `clear`. To improve the
UX and fulfill the user expectations, the default has been adjusted to
work the same as in other shells by clearing the scrollback buffer. For
edge cases where someone depends on the current behavior of keeping the
scrollback, a `-k` option has been added.

# User-Facing Changes
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helps us keep track of breaking changes. -->

- Improve the UX of `clear` by changing the default behavior to the same
as other popular shells, i.e clear scrollback by default.
- Remove `-a --all` flag, make it the default behavior to clear the
scrollback
- Add `-k --keep-scrollback` flag for backward compat to keep the
scrollback buffer

# Tests + Formatting
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This is a simple change flipping the flag and default behavior, no tests
should be needed.

# After Submitting
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- [ ] update the `clear` command docs

---------

Co-authored-by: Douglas <32344964+NotTheDr01ds@users.noreply.github.com>
Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>

crates/nu-command/src/platform/clear.rs

@@ -19,14 +19,18 @@ impl Command for Clear {
         "Clear the terminal."
     }
 
+    fn extra_description(&self) -> &str {
+        "By default clears the current screen and the off-screen scrollback buffer."
+    }
+
     fn signature(&self) -> Signature {
         Signature::build("clear")
             .category(Category::Platform)
             .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .switch(
-                "all",
+                "keep-scrollback",
-                "Clear the terminal and its scroll-back history",
+                "Do not clear the scrollback history",
-                Some('a'),
+                Some('k'),
             )
     }
 
@@ -37,9 +41,9 @@ impl Command for Clear {
         call: &Call,
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
-        let clear_type: ClearType = match call.has_flag(engine_state, stack, "all")? {
+        let clear_type: ClearType = match call.has_flag(engine_state, stack, "keep-scrollback")? {
-            true => ClearType::Purge,
+            true => ClearType::All,
-            _ => ClearType::All,
+            _ => ClearType::Purge,
         };
         std::io::stdout()
             .queue(ClearCommand(clear_type))?
@@ -57,8 +61,8 @@ impl Command for Clear {
                 result: None,
             },
             Example {
-                description: "Clear the terminal and its scroll-back history",
+                description: "Clear the terminal but not its scrollback history",
-                example: "clear --all",
+                example: "clear --keep-scrollback",
                 result: None,
             },
         ]