Add config for trimming trailing whitespace and newlines on write

These match the equivalent options in VSCode. `trim_trailing_whitespace`
is also the name used by EditorConfig.

* `trim-final-newlines` trims any extra line endings after the final one
* `trim-trailing-whitespace` trims any trailing whitespace (but not
  empty lines)

helix-term/tests/test/commands/write.rs

@@ -420,6 +420,50 @@ async fn test_write_utf_bom_file() -> anyhow::Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread")]
+async fn test_write_trim_trailing_whitespace() -> anyhow::Result<()> {
+    let mut file = tempfile::NamedTempFile::new()?;
+    let mut app = helpers::AppBuilder::new()
+        .with_config(Config {
+            editor: helix_view::editor::Config {
+                trim_trailing_whitespace: true,
+                ..Default::default()
+            },
+            ..Default::default()
+        })
+        .with_file(file.path(), None)
+        .with_input_text("#[f|]#oo      \n\n \nbar      ")
+        .build()?;
+
+    test_key_sequence(&mut app, Some(":w<ret>"), None, false).await?;
+
+    helpers::assert_file_has_content(&mut file, &LineFeedHandling::Native.apply("foo\n\n\nbar"))?;
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread")]
+async fn test_write_trim_final_newlines() -> anyhow::Result<()> {
+    let mut file = tempfile::NamedTempFile::new()?;
+    let mut app = helpers::AppBuilder::new()
+        .with_config(Config {
+            editor: helix_view::editor::Config {
+                trim_final_newlines: true,
+                ..Default::default()
+            },
+            ..Default::default()
+        })
+        .with_file(file.path(), None)
+        .with_input_text("#[f|]#oo\n \n\n\n")
+        .build()?;
+
+    test_key_sequence(&mut app, Some(":w<ret>"), None, false).await?;
+
+    helpers::assert_file_has_content(&mut file, &LineFeedHandling::Native.apply("foo\n \n"))?;
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread")]
 async fn test_write_insert_final_newline_added_if_missing() -> anyhow::Result<()> {
     let mut file = tempfile::NamedTempFile::new()?;