[4/?] Add ability to run code examples in the playground: Get pony snippets tested with ponyc at CI time

[shaedrich]

Part of #340

• Where do we put the expectations for stdOut, stdErr, and exitCode? Because if we don't do that, only snippets that print to stdOut could be tested. docstring as frontmatter perhaps?

Chapter	Snippet	stdOut	stdErr	error line	exitCode
Getting started ▸ Hello world	hello-world-main.pony	Hello, world!	n/a	n/a	0
Types ▸ Classes	classes-swap-values-sugar.pony	a = 1, b = 2 a = 2, b = 1	n/a	n/a	0
Types ▸ Classes	classes-swap-values.pony	a = 1, b = 2 a = 2, b = 1	n/a	n/a	0
Types ▸ Classes	classes-wombat.pony	Your default wombat's name is "Fantastibat" Your hungry wombat's name is "Nomsbat"	n/a	n/a	0
Types ▸ Primitives	primitives-doors.pony	Is door open? false 2 + 3 = 5	n/a	n/a	0
Types ▸ Actors	actors-sequential.pony	This is printed first This is printed last	n/a	n/a	0
Types ▸ Traits and interfaces	traits-and-interfaces-multiple-traits.pony	Their name is "Bob" and they are not bald	n/a	n/a	0
Types ▸ Traits and interfaces	traits-and-interfaces-nested-traits.pony	Their name is "Bob" and they are not bald	n/a	n/a	0
Types ▸ Traits and interfaces	traits-and-interfaces-nominal-and-structural-subtyping.pony	Their name is "Bob" Their name is "Larry"	n/a	n/a	0
Types ▸ Traits and interfaces	traits-and-interfaces-nominal-subtyping-in-pony.pony	Their name is "Larry"	n/a	n/a	0
Types ▸ Traits	traits-and-interfaces-open-world-typing.pony	size: 2, space: 8 size: 2, space: 8 size: 3, space: 8 size: 3, space: 4	n/a	n/a	0
Types ▸ Traits and interfaces	traits-and-interfaces-private-methods.pony	n/a	interfaces can't have private methods, only traits can	9:11	1?
Types ▸ Traits and interfaces	traits-and-interfaces-trait.pony	Their name is "Bob"	n/a	n/a	0
Types ▸ Type aliases	type-aliases-enumerations-apply.pony	red: #FF0000FF green: #FF00FF blue: #FFFF	n/a	n/a	0
Types ▸ Type aliases	type-aliases-complex-types-trait.pony	Bob, aged 42, feels "Great!"	n/a	n/a	0

[shaedrich]

That's the current output:

Errors:

Successful:

[jemc]

As discussed before and in other PRs, we're mainly concerned with the checking whether something compiles or not (and a regular expression match for the error message in the expected-not-to-compile case).

So that approach drastically reduces how much of this expectation information we need to track - we essentially don't need to track anything at all for examples which are expected to compile, and for those which are expected to have a compilation error, we only need a string / regular expression to match against the error output.

As for the question of where these expectations should be tracked, I think if they were in Pony docstring of the source files themselves, that would risk confusion for the reader of these snippets in the playground (they might likely ask themselves if these expectations were actually of a part of Pony the language). So that leaves basically two options:

1. A central file somewhere with a list of expected-not-to-compile sample names, each mapped to a string with a pattern to check for in the compilation error.
2. Decentralized files - each file having the same name as the sample it maps to, but with some extra suffix (such as .error.txt) and the content of the file is the expected error pattern to match.

We're not strongly opinionated about these two approaches, but I have a slight preference for the latter.