feat(config): add project configuration

[akinsho]

Implements #79
cc @sidlatau here's the branch I mentioned.

Currently the workflow is, a user creates an exrc file ie. a .nvim.lua or a .nvimrc and they add something like

require('flutter-tools').setup_project({
  device = "iPhone10",
  flavour = "Production",
})

-- Or
require('flutter-tools').setup_project({
	{
  		device = "iPhone10",
	    flavour = "Production",
		dart_define = {
			API_URL = "blah/thing.com",
		}
	},
	{
		device = "macOS",
		flavour = "Desktop",
	}
})

I've only implemented support for the device key atm but the others could be added eventually. The plan won't be to support everything immediately, but just to get a system in place for this PR.

[sidlatau]

I want to add more details, about what problem I want to solve with project config: I have a project with two flavors and dart-defines(eg. DEV and PROD flavors). Now I have keymaps set up for each flavor in my dotfiles (FlutterRun --flavor...). But that is only related to the project, so it nice candidate for the project .nvim.lua configuration. So there are two configurations and I want an easy way to switch between them (maybe with the ability in lualine to see which configuration is currently selected).
Am I correct that in this PR the configuration will be set only on vim start?

[akinsho]

@sidlatau yeah currently the idea was just picking one of the configs on startup but that's somewhat arbitrary since I don't have that use case. It can easily store both configs and then when you run start we can check if you have a project config and show the selection prompt at that stage? would that solve the runtime issue?

[sidlatau]

Yes, that may work. I was also thinking about a separate command for setting configuration. But configuration should be set before running FlutterRun. So maybe there is no point in adding separate command if that is a part of the process of starting the flutter app.

[akinsho]

@sidlatau yeah the configuration is IMO similar to a launch.json or something it's something that you can change over time but isn't intended to be some super dynamic thing that needs changing with a command presumably there are standard ways to run your project and those are fairly consistent and not things that are changing super often.

[sidlatau]

Could support for flavor and dart_define be added to this PR too? I think these two adds most of the value plus then I would be able to help with testing this functionality with my project setup.

[sidlatau]

The functionality works with my project configuration 👍🏻
I think mentioning in README/docs is missing, and it will be LGTM

[sidlatau]

One more thing: if the app is already started and I run FlutterRun, then I get the selector and then the info message:

Info  14:57:31 notify.info Flutter tools Flutter is already running!

Maybe it is possible to check if Flutter is already running before showing the selector?