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/highlights/treesitter.lua

@@ -9,7 +9,7 @@ function M.setup(colors, config)
         -- @variable                       various variable names
         ["@variable"] = { fg = theme.ui.fg },
         -- @variable.builtin (Special)     built-in variable names (e.g. `this`, `self`)
-        ["@variable.builtin"] = { fg = theme.syn.special2, italic = not config.disableItalics },
+        ["@variable.builtin"] = { fg = theme.syn.special2, italic = config.italics },
         -- @variable.parameter             parameters of a function
         ["@variable.parameter"] = { fg = theme.syn.parameter },
         -- @variable.parameter.builtin     special parameters (e.g. `_`, `it`)
@@ -80,7 +80,7 @@ function M.setup(colors, config)
         ["@keyword.return"] = vim.tbl_extend(
             "force",
             { fg = theme.syn.special3 },
-            config.disableItalics and {} or config.keywordStyle
+            not config.italics and {} or config.keywordStyle
         ),
         -- @keyword.debug          keywords related to debugging
         -- @keyword.exception      keywords related to exceptions (e.g. `throw`, `catch`)