Add Text.Megaparsec.Error.errorBundlePrettyForGhcPreProcessors

[sol]

If you use errorBundlePretty when writing a GHC pre-processor then error output does not look pretty:

To address this, this PR adds errorBundlePrettyForGhcPreProcessors, which results in:

Details:

1. When a GHC pre-processor fails, then GHC parses source locations and error messages from the pre-processor's output.
2. Multi-line error messages have to be indented, otherwise GHC won't parse them properly.
3. When GHC pretty-prints error message it indents multi-line error messages by four spaces, but it does not strip any whitespace from parsed error messages before pretty-printing them.

As a consequence, the best you can achieve for a pre-processor is to have multi-line error messages indented by five spaces. To achieve this you have to produce error messages of the form:

src/Foo.hs:23:42: foo
 bar
 baz

That is:

1. The first line of the message has to be on the same same line as the source location separated by exactly one space.
2. Each subsequent line has to be indented by one space.

Note that single-line error message should be of the form:

src/Foo.hs:23:42:foo

(no space between the last : and the error message, otherwise you will end up with two spaces in the pretty-printed output)

[mrkkrp]

Thanks!