feat(diagnostics): add vim.diagnostic.get_namespaces (#15866)

Many vim.diagnostic functions expect the user to pass in a namespace id.

This PR allows the user to list active diagnostic namespaces:

```lua
:lua print(vim.inspect(vim.diagnostic.get_namespaces()))

{
  [7] = {
    name = "vim.lsp.client-1",
    opts = {},
    sign_group = "vim.diagnostic.vim.lsp.client-1"
  }
}
```

1 files changed, 7 insertions, 0 deletions

diff --git a/runtime/lua/vim/diagnostic.lua b/runtime/lua/vim/diagnostic.lua
index c9119b4c42..35badf8a02 100644
--- a/runtime/lua/vim/diagnostic.lua
+++ b/runtime/lua/vim/diagnostic.lua
@@ -646,6 +646,13 @@ function M.set(namespace, bufnr, diagnostics, opts)
   vim.api.nvim_command("doautocmd <nomodeline> User DiagnosticsChanged")
 end
 
+--- Get current diagnostic namespaces.
+---
+---@return table A list of active diagnostic namespaces |vim.diagnostic|.
+function M.get_namespaces()
+  return vim.deepcopy(all_namespaces)
+end
+
 --- Get current diagnostics.
 ---
 ---@param bufnr number|nil Buffer number to get diagnostics from. Use 0 for