feat: make struct name and path optional for server functions

[gbj]

This PR changes the behavior of the #[server] macro to do two things:

1. makes the struct name argument optional and defaults to generating a PascalCase version of the snake_case function name
2. changes the default path-prefix argument from / to /api

As a result, you can now write

#[server(DeleteTodo, "/api")]
pub async fn delete_todo(id: u16) -> Result<(), ServerFnError> {

as

#[server]
pub async fn delete_todo(id: u16) -> Result<(), ServerFnError> {

Generating struct names is arguably bad proc-macro hygiene, which is why I didn't do it in the first place. The #[component] macro also does this when it generates a type name for ComponentProps, but this type name doesn't need to be referred to by the user. Server function types are more often used explicitly:

// uh where does this type name come from?
let action = create_server_action::<DeleteTodo>();

Still, I think this is probably better. And rust-analyzer is, of course, smart enough to bring you directly to the #[server] macro for the correct function if you "Go to Definition" on DeleteTodo in this example.

Re: / vs /api, mounting server functions at / by default was probably never the right idea. Our starter templates all default to /api. This will be a breaking change for anyone who was relying on that default mount point, but is easily addressed by moving that to /api.