add "default" table theme (#11072)

# Description

This PR fixes a minor bug that prevented this command from running.
```nushell
table --list | each {|r| print ($r); print (ls | first 3 | table --theme $r)}
```
Here's the output now of the first few themes.

![image](https://github.com/nushell/nushell/assets/343840/21bc8942-5106-4b6a-8905-e90d6cb9a153)

It prevented it from running because "default" wasn't a real table
theme. Now "default" is a synonym of rounded.

Also tweaked the error message when a bad theme name is provided.

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crates/nu-protocol/src/config/table.rs

@@ -11,6 +11,7 @@ pub enum TableMode {
     Compact,
     WithLove,
     CompactDouble,
+    Default,
     #[default]
     Rounded,
     Reinforced,
@@ -35,6 +36,7 @@ impl FromStr for TableMode {
             "compact" => Ok(Self::Compact),
             "with_love" => Ok(Self::WithLove),
             "compact_double" => Ok(Self::CompactDouble),
+            "default" => Ok(Self::Rounded),
             "rounded" => Ok(Self::Rounded),
             "reinforced" => Ok(Self::Reinforced),
             "heavy" => Ok(Self::Heavy),
@@ -60,6 +62,7 @@ impl ReconstructVal for TableMode {
                 TableMode::Compact => "compact",
                 TableMode::WithLove => "with_love",
                 TableMode::CompactDouble => "compact_double",
+                TableMode::Default => "rounded",
                 TableMode::Rounded => "rounded",
                 TableMode::Reinforced => "reinforced",
                 TableMode::Heavy => "heavy",