feat: diffview.nvim integration (#359)

This integration fixes the colourscheme for diffview.nvim
and matches it to that of GitHub code diff view.

Optimized for and tested with dark mode.

lua/base46/integrations/diffview.lua

@@ -0,0 +1,13 @@
+local colors = require("base46").get_theme_tb "base_30"
+local mix_col = require("base46.colors").mix
+
+local highligths = {
+  DiffviewDiffAdd = { bg = mix_col(colors.green, colors.black, 85) },
+  DiffviewDiffText = { bg = mix_col(colors.green, colors.black, 70) },
+  DiffviewDiffChange = { bg = mix_col(colors.green, colors.black, 85) },
+  DiffviewDiffDelete = { bg = mix_col(colors.red, colors.black, 70) },
+  DiffviewDiffDeleteDim = { bg = mix_col(colors.red, colors.black, 85) },
+  DiffviewDiffAddAsDelete = { bg = mix_col(colors.red, colors.black, 85) },
+}
+
+return highligths