Fixes String iterpolator with \"

Fixes scala/bug#6476

SIP-11 specification says

```
interpolatedString ::= alphaid `"` {printableChar \ (`"` | `$`) | escape} `"`
```

My reading of this is that the scanner should support the character sequence `\"`, but pass them as is. The Standard (s) interpolator would take care of the evaluation of `\"` as `"`.
This would fix the long standing issue of people trying to write `s"a=\"$a\""` and running into mysterious `';' expected but string literal found.`.

This change should be relatively safe since neither `s"\"foo\""` nor `s"foo\"` previously compiled.

src/compiler/scala/tools/nsc/ast/parser/Scanners.scala

@@ -907,6 +907,20 @@ trait Scanners extends ScannersCommon {
           setStrVal()
           token = STRINGLIT
         }
+      } else if (ch == '\\') {
+        if (multiLine) {
+          putChar(ch)
+          nextRawChar()
+          getStringPart(multiLine)
+        } else {
+          putChar(ch)
+          nextRawChar()
+          if (ch == '"') {
+            putChar(ch)
+            nextRawChar()
+          }
+          getStringPart(multiLine)
+        }
       } else if (ch == '$') {
         nextRawChar()
         if (ch == '$') {

test/files/neg/t8266-invalid-interp.check

@@ -1,10 +1,4 @@
-t8266-invalid-interp.scala:4: error: Trailing '\' escapes nothing.
+t8266-invalid-interp.scala:4: error: unclosed string literal
     f"a\",
-       ^
-t8266-invalid-interp.scala:5: error: invalid escape '\x' not one of [\b, \t, \n, \f, \r, \\, \", \', \uxxxx] at index 1 in "a\xc". Use \\ for literal \.
-    f"a\xc",
-       ^
-t8266-invalid-interp.scala:7: error: invalid escape '\v' not one of [\b, \t, \n, \f, \r, \\, \", \', \uxxxx] at index 1 in "a\vc". Use \\ for literal \.
-    f"a\vc"
-       ^
-3 errors
+      ^
+1 error

test/files/run/string-interpolator.check

@@ -0,0 +1 @@
+"Hello", Alice

test/files/run/string-interpolator.scala

@@ -0,0 +1,6 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    val person = "Alice"
+    println(s"\"Hello\", $person")
+  }
+}