feat: add tool args

[gtbono]

Hello!

When running Flutter tests in Emacs with dart-lsp, I used the Code Lens feature to click on Test and Debug lens on top of my tests.

But for an application I was working on, I needed to pass the --no-sound-null-safety argument to the Flutter CLI when debugging my tests. To make the Test lens run, I needed to add the following configuration in my Emacs:

(setq lsp-dart-test-extra-args '("--no-sound-null-safety"))

But it didn't work for the Debug lens, in the dart-lsp codebase, I can see that the lsp-dart-test-extra-args option sends the arguments to Dart's vmAdditionalArgs, but looking in the dartcode.org website https://dartcode.org/docs/launch-configuration for debugging I needed to pass the argument to toolArgs for it to work, and there is currently no way to do it.

So this PR adds a variable (is variable the right way to call this? I'm an Emacs newbie 😅 ) called lsp-dart-dap-tool-args that can be used like this:

(setq lsp-dart-dap-tool-args ["--no-sound-null-safety"])

With this turned on, the Debug lens work with the null safety argument being passed.

This is my first contribution, so I don't know how things work, do I need to bump the changelog too?

[ericdallo]

Looks good, thanks!