feat\!: replace disableItalics with italics config option

Replace the negative boolean `disableItalics` with positive `italics` option for better API ergonomics. This affects all italic styling throughout the theme including comments, keywords, and plugin highlights.

BREAKING CHANGE: Users must update their configuration from `disableItalics = true` to `italics = false`.

lua/kanso/init.lua

@@ -9,6 +9,7 @@ local M = {}
 ---@class KansoConfig
 M.config = {
     bold = true,
+    italics = true,
     undercurl = true,
     commentStyle = { italic = true },
     functionStyle = {},
@@ -18,7 +19,6 @@ M.config = {
     transparent = false,
     dimInactive = false,
     terminalColors = true,
-    disableItalics = false,
     colors = { theme = { zen = {}, pearl = {}, ink = {}, all = {} }, palette = {} },
     ---@type fun(colors: KansoColorsSpec): table<string, table>
     overrides = function()