Add command to translate some inline type annotations to RBS comments

[Morriar]

This PR add translation of T.let when found in an assign to RBS comments.

This:

a = T.let(nil, T.nilable(Integer))
b = ::T.let([], T::Array[Integer])

is translated into:

a = nil #: Integer?
b = [] #: Array[Integer]

Note that we only support translation in assigns for local, global, instance, class variables, constants and constant paths.

Annotations not in an assign node are ignored:

ignored = foo(T.let(nil, T.nilable(Integer))
T.let(also_ignored, T.nilable(Integer))

We also try our best to make the resulting code pass style checks and reindent properly:

a = T.let(
  [
    1, 
    2, 
    3
  ],
  T::Array[Integer]
)

is thus rewrote as:

a = [
  1, 
  2, 
  3
] #: Array[Integer]

This is still experimental as I'm sure we're missing a bunch of edge-cases but that should be enough to get us started.

Here are two examples running this tool on other open-source projects:

• https://github.com/Shopify/rbi/pull/new/at-rbs-annotations
• https://github.com/Shopify/tapioca/pull/new/at-rbs-annotations

[Morriar]

Fixed review comments, also enhanced the placement of the comment to be close to the \n so we can support cases like this:

(x = T.let(y, String))
# becomes
(x = y) #: String

x = T.let(y, String) if z
# becomes
x = y if z #: String