add @stipple_precompile for easier precompilation of user apps

[hhaensel]

This PR strongly simplifies the writing of a precompile section for Stipple apps.

Stipple.jl itself now uses the new macro @stipple_precompile to precompile functions for typical usage. The previous defintion has come down to.

@stipple_precompile begin
    ui() = [cell("hello"), row("world"), htmldiv("Hello World")]

    @app PrecompileApp begin
        @in demo_i = 1
        @out demo_s = "Hi"

        @onchange demo_i begin
        println(demo_i)
        end
    end

    route("/") do 
        model = Stipple.ReactiveTools.@init PrecompileApp
        page(model, ui) |> html
    end

    precompile_get("/")
    deps_routes(core_theme = true)
    precompile_get(Genie.Assets.asset_path(assets_config, :js, file = "stipplecore"))
end

and all previous of spinning up a server which doesn't spill the precompilation output and checking whether server requests should be carried out during precompilation are now handled by the macro.
HTTP.get simply needs to be replaced by precompile_get()
The previous setting ENV["STIPPLE_PRECOMPILE_GET"] has been replaced by ENV["STIPPLE_PRECOMPILE_REQUESTS"] but is currently kept for backward compatibility.