Add rustacean.nvim and dap plugins

lua/custom/plugins/dap.lua

@@ -0,0 +1 @@
+return { 'mfussenegger/nvim-dap' }

lua/custom/plugins/rustacean.lua

@@ -0,0 +1,9 @@
+return {
+  'mrcjkb/rustaceanvim',
+  version = '^4', -- Recommended
+  lazy = false, -- This plugin is already lazy
+}
+
+-- nvim-lspconfig.rust_analyzer has been setup.
+-- This will likely lead to conflicts with the rustaceanvim LSP client.
+-- See ':h rustaceanvim.mason'