Backslash escapes shouldn't be recognized in raw strings

[tplunket]

Triple-quoted strings in Kotlin do not do any escape processing so backslash-character should not be interpreted as an escape sequence.

E.g. my TeamCity config contains lines such as

        scriptContent = """tools\bin\make -C workdir myapp"""

...and the backslash-b is highlighted as if it is valid while backslash-m is highlighted as an error. Neither of these should be highlight.

I believe that the fix is to simply remove the ,ktSpecialChar,ktSpecialCharError bit from the end of the line in syntax/kotlin.vim for ktString start='"""' but I'm not sure if that's entirely correct.

I reference the information about "raw strings" on https://devtut.github.io/kotlin/strings.html#string-literals