added shfmt for shell scripts

Format sh/bash files with shfmt via conform and install it through
mason-tool-installer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lua/holger/plugins/formatting.lua

@@ -15,6 +15,8 @@ return {
 				liquid = { "prettier" },
 				lua = { "stylua" },
 				python = { "isort", "black" },
+				sh = { "shfmt" },
+				bash = { "shfmt" },
 			},
 			format_on_save = {
 				lsp_fallback = true,

lua/holger/plugins/lsp/mason.lua

@@ -34,6 +34,7 @@ return {
 				"stylua", -- lua formatter
 				"isort", -- python formatter
 				"black", -- python formatter
+				"shfmt", -- shell formatter
 				"pylint",
 			},
 		},