Wire allows unquoted imports

[JesseWeinstein]

Wire allows .proto files to contain unquoted imports, like import Foo.proto; while the spec requires them to be quoted:

strLit = strLitSingle { strLitSingle }
strLitSingle = ( "'" { charValue } "'" ) | ( '"' { charValue } '"' )

import = "import" [ "weak" | "public" ] strLit ";"

This is because ProtoParser.kt uses reader.readString rather than readQuotedString.

It would be nice to at least have an option to refuse these, for consistency with the spec and other tooling.