widgets: add LineNumbers widget

The LineNumbers widget draws vertical list of numbers.
This can be used as a line number bar for a text editor for example.

src/widgets.zig

@@ -7,3 +7,4 @@ pub const Table = @import("widgets/Table.zig");
 pub const TextInput = @import("widgets/TextInput.zig");
 pub const nvim = @import("widgets/nvim.zig");
 pub const ScrollView = @import("widgets/ScrollView.zig");
+pub const LineNumbers = @import("widgets/LineNumbers.zig");

src/widgets/LineNumbers.zig

@@ -0,0 +1,54 @@
+const std = @import("std");
+const vaxis = @import("../main.zig");
+
+const digits = "0123456789";
+
+num_lines: usize = std.math.maxInt(usize),
+highlighted_line: usize = 0,
+style: vaxis.Style = .{ .dim = true },
+highlighted_style: vaxis.Style = .{ .dim = true, .bg = .{ .index = 0 } },
+
+pub fn extractDigit(v: usize, n: usize) usize {
+    return (v / (std.math.powi(usize, 10, n) catch unreachable)) % 10;
+}
+
+pub fn numDigits(v: usize) usize {
+    return switch (v) {
+        0...9 => 1,
+        10...99 => 2,
+        100...999 => 3,
+        1000...9999 => 4,
+        10000...99999 => 5,
+        100000...999999 => 6,
+        1000000...9999999 => 7,
+        10000000...99999999 => 8,
+        else => 0,
+    };
+}
+
+pub fn draw(self: @This(), win: vaxis.Window, y_scroll: usize) void {
+    for (1 + y_scroll..self.num_lines) |line| {
+        if (line - 1 >= y_scroll +| win.height) {
+            break;
+        }
+        const highlighted = line == self.highlighted_line;
+        const num_digits = numDigits(line);
+        for (0..num_digits) |i| {
+            const digit = extractDigit(line, i);
+            win.writeCell(win.width -| (i + 2), line -| (y_scroll +| 1), .{
+                .char = .{
+                    .width = 1,
+                    .grapheme = digits[digit .. digit + 1],
+                },
+                .style = if (highlighted) self.highlighted_style else self.style,
+            });
+        }
+        if (highlighted) {
+            for (num_digits + 1..win.width) |i| {
+                win.writeCell(i, line -| (y_scroll +| 1), .{
+                    .style = if (highlighted) self.highlighted_style else self.style,
+                });
+            }
+        }
+    }
+}