feat(diagnostic): add default mappings for diagnostics

Add mappings for goto_next, goto_prev, and open_float.

runtime/doc/diagnostic.txt

@@ -340,6 +340,20 @@ Example: >lua
       end,
     })
 <
+==============================================================================
+MAPS                                                    *diagnostic-maps*
+
+                                                       *]d-default*
+]d                        Goto the next diagnostic in the buffer.
+                            |vim.diagnostic.goto_next()|
+                                                       *[d-default*
+[d                        Goto the previous diagnostic in the buffer.
+                            |vim.diagnostic.goto_prev()|
+
+CTRL-W CTRL-D                                          *CTRL-W_d-default*
+CTRL-W d                  Open a floating window for diagnostics under
+                          the cursor. |vim.diagnostic.open_float()|.
+
 ==============================================================================
 Lua module: vim.diagnostic                                    *diagnostic-api*
 

runtime/doc/vim_diff.txt

@@ -137,6 +137,9 @@ of these in your config by simply removing the mapping, e.g. ":unmap Y".
 - * |v_star-default|
 - gc |gc-default| |v_gc-default| |o_gc-default|
 - gcc |gcc-default|
+- ]d |]d-default|
+- [d |[d-default|
+- <C-W>d |CTRL-W_d-default|
 - Nvim LSP client defaults |lsp-defaults|
   - K |K-lsp-default|
 

runtime/lua/vim/_defaults.lua

@@ -126,7 +126,9 @@ do
     end, { desc = gx_desc })
   end
 
-  --- Default maps for built-in commenting
+  --- Default maps for built-in commenting.
+  ---
+  --- See |gc-default| and |gcc-default|.
   do
     local operator_rhs = function()
       return require('vim._comment').operator()
@@ -143,6 +145,35 @@ do
     end
     vim.keymap.set({ 'o' }, 'gc', textobject_rhs, { desc = 'Comment textobject' })
   end
+
+  --- Map [d and ]d to move to the previous/next diagnostic. Map <C-W>d to open a floating window
+  --- for the diagnostic under the cursor.
+  ---
+  --- See |[d-default|, |]d-default|, and |CTRL-W_d-default|.
+  do
+    vim.keymap.set('n', ']d', function()
+      vim.diagnostic.goto_next({ severity = 'auto', float = false })
+    end, {
+      desc = 'Jump to the next diagnostic with the highest severity',
+    })
+
+    vim.keymap.set('n', '[d', function()
+      vim.diagnostic.goto_prev({ severity = 'auto', float = false })
+    end, {
+      desc = 'Jump to the previous diagnostic with the highest severity',
+    })
+
+    vim.keymap.set('n', '<C-W>d', function()
+      vim.diagnostic.open_float({ border = 'rounded' })
+    end, {
+      desc = 'Open a floating window showing diagnostics under the cursor',
+    })
+
+    vim.keymap.set('n', '<C-W><C-D>', '<C-W>d', {
+      remap = true,
+      desc = 'Open a floating window showing diagnostics under the cursor',
+    })
+  end
 end
 
 --- Default menus