separate newline style from whitespace style

New "ui.virtual.newline" style is used for newline characters.
Will fallback on "ui.virtual.whitespace" if not defined.

Useful when newline render is 'after-whitespace', since in this case
you want it to stand out, whereas other whitespace characters would
need to blend in the background.

It was also conflicting with my theme where string literals are
underlined, and the "ui.virtual.whitespace" fg color was applied
on the underline on whitespaces inside strings.

Example theme:
    "ui.virtual.whitespace" = { fg = "gray", modifiers = ["dim"] }
    "ui.virtual.newline" = { fg = "red", modifiers = ["bold"] }

helix-term/src/ui/document.rs

@@ -364,6 +364,7 @@ pub struct TextRenderer<'a> {
     pub surface: &'a mut Surface,
     pub text_style: Style,
     pub whitespace_style: Style,
+    pub newline_style: Style,
     pub indent_guide_char: String,
     pub indent_guide_style: Style,
     pub newline: NewlineRendering,
@@ -451,6 +452,9 @@ impl<'a> TextRenderer<'a> {
             tab,
             virtual_tab,
             whitespace_style: theme.get("ui.virtual.whitespace"),
+            newline_style: theme
+                .try_get("ui.virtual.newline")
+                .unwrap_or_else(|| theme.get("ui.virtual.whitespace")),
             indent_width,
             starting_indent: col_offset / indent_width as usize
                 + (col_offset % indent_width as usize != 0) as usize
@@ -484,7 +488,11 @@ impl<'a> TextRenderer<'a> {
         // TODO is it correct to apply the whitespace style to all unicode white spaces?
         let mut style = grapheme_style.syntax_style;
         if is_whitespace {
-            style = style.patch(self.whitespace_style);
+            style = if grapheme == Grapheme::Newline {
+                style.patch(self.newline_style)
+            } else {
+                style.patch(self.whitespace_style)
+            };
         }
         style = style.patch(grapheme_style.overlay_style);