change default embedded font to JetBrains Mono

Fixes #991

This changes the default embedded font from Fira to JetBrains Mono. This
only affects users who don't specify or font AND Ghostty can't find
another default font to use on their system.

This is one part an aesthetic choice: I've grown to personally like
JetBrains Mono more than Fira, and I think I have the right to change
the defaults ;)

But this is also partly because of #991: FiraCode contains glyphs for
symbolic ranges. This may not be Fira official... I think I may have
used a Nerd font patched one and that may be the issue. So I don't want
to blame the Fira font project. BUT, we have to replace the ttf in our
project and since I've been meaning to switch to JB Mono I just did that
now.

src/Surface.zig

@@ -3021,7 +3021,7 @@ fn showDesktopNotification(self: *Surface, title: [:0]const u8, body: [:0]const
     } else log.warn("runtime doesn't support desktop notifications", .{});
 }
 
-pub const face_ttf = @embedFile("font/res/FiraCode-Regular.ttf");
-pub const face_bold_ttf = @embedFile("font/res/FiraCode-Bold.ttf");
+pub const face_ttf = @embedFile("font/res/JetBrainsMono-Regular.ttf");
+pub const face_bold_ttf = @embedFile("font/res/JetBrainsMono-Bold.ttf");
 pub const face_emoji_ttf = @embedFile("font/res/NotoColorEmoji.ttf");
 pub const face_emoji_text_ttf = @embedFile("font/res/NotoEmoji-Regular.ttf");