refactor: move vim.lsp.diagnostic to vim.diagnostic

This generalizes diagnostic handling outside of just the scope of LSP.
LSP clients are now a specific case of a diagnostic producer, but the
diagnostic subsystem is decoupled from the LSP subsystem (or will be,
eventually).

More discussion at [1].

[1]: https://github.com/neovim/neovim/pull/15585

scripts/gen_vimdoc.py

@@ -187,6 +187,23 @@ CONFIG = {
         'module_override': {},
         'append_only': [],
     },
+    'diagnostic': {
+        'mode': 'lua',
+        'filename': 'diagnostic.txt',
+        'section_start_token': '*diagnostic-api*',
+        'section_order': [
+            'diagnostic.lua',
+        ],
+        'files': os.path.join(base_dir, 'runtime/lua/vim/diagnostic.lua'),
+        'file_patterns': '*.lua',
+        'fn_name_prefix': '',
+        'section_name': {'diagnostic.lua': 'diagnostic'},
+        'section_fmt': lambda _: 'Lua module: vim.diagnostic',
+        'helptag_fmt': lambda _: '*diagnostic-api*',
+        'fn_helptag_fmt': lambda fstem, name: f'*vim.{fstem}.{name}()*',
+        'module_override': {},
+        'append_only': [],
+    },
     'treesitter': {
         'mode': 'lua',
         'filename': 'treesitter.txt',