Adding a new user command together with help section on user commands

[Geerayef]

Hello!

I've added a new user command for clearing all set breakpoints and a help section on user commands provided by nvim-dap in my local clone of the project.

The use case, in my personal configuration, for nvim-dap's user commands is when making key mappings, as the following example:

vim.keymap.set("n", "<M-d>b", vim.cmd.DapClearBreakpoints, { desc = "Clear all breakpoints." })

The help section that I wrote simply lists all the available user commands with a short, one-sentence explanation of what they do.

I wanted to verify with you @mfussenegger if that's something you'd like to have added to the plugin? If yes, I can make a PR.

Thank you for the incredible work you've done with nvim-dap!