add(style): merge function to merge styles from different styles together

Merging may overwrite changes of the current style with the other style
if that style differs from the default value.

src/terminal/Style.zig

@@ -113,6 +113,22 @@ reverse: bool = false,
 invisible: bool = false,
 strikethrough: bool = false,
 
+/// Merge _other_ `Style` to _this_ style and overwrite _this_ `Style`'s value
+/// if the _other_ value differs from the default value.
+pub fn merge(this: *@This(), other: @This()) void {
+    if (other.fg != .default) this.fg = other.fg;
+    if (other.bg != .default) this.bg = other.bg;
+    if (other.ul != .default) this.ul = other.ul;
+    if (other.ul_style != .off) this.ul_style = other.ul_style;
+    if (other.bold == false) this.bold = other.bold;
+    if (other.dim == false) this.dim = other.dim;
+    if (other.italic == false) this.italic = other.italic;
+    if (other.blink == false) this.blink = other.blink;
+    if (other.reverse == false) this.reverse = other.reverse;
+    if (other.invisible == false) this.invisible = other.invisible;
+    if (other.strikethrough == false) this.strikethrough = other.strikethrough;
+}
+
 fn start(this: @This(), writer: anytype) !void {
     // foreground
     switch (this.fg) {