Implement translation for PM_BEGIN_NODE

[egiurleo]

Closes #47

Motivation

Translates PM_BEGIN_NODEs from Prism to Sorbet.

These represent begin blocks, e.g.

begin
  # ...
rescue
  # ...
end

Note that I haven't implemented support for rescue (#160), ensure (#86) or else because they're not needed for parsing the RBI gem.

Test plan

See included automated tests.

[amomchilov]

Could you open an issue to track the translation of else_clause?

The rescue and ensure are their own node types, so we'll implement those in #160 and #86.

[egiurleo]

@amomchilov Good point re: else. I actually added a note to our rescue ticket because the else clause works together with rescue.