Currently, the feature file parser always resolves included files,
parses their content and inserts it in the resulting AST. The original
`include` statement is lost.
This commit introduces an option to not follow inclusions. Instead, the
output AST will contain an `include` statement. This allows to process a
feature file on its own, and allows to round-trip it.
For example in glyphsLib, when going from a UFO to a .glyphs file, a
UFO feature file will be sliced up into Glyphs.app classes (e.g. a
GSFeaturePrefix with code `include(../family.fea);`) and when going back
from .glyphs to UFO, the feature file will be patched back together.
89979dea04