Add test for {interpolation} after two double quotes

This is an edge case of how the lexer changes behavior when
reading string literals versus reading unquoted source code.
Three double quotes would begin a multi-line string literal,
but just two are still ambiguous, so it has to `peek()` to
see whether it's inside a multi-line string literal or
outside an empty regular string.

test/asm/interpolation-after-string.asm

@@ -0,0 +1,8 @@
+def hello = $11
+def goodbye = $22
+
+def greeting equs "hello"
+println ""{greeting}
+
+def string equs "\"goodbye\""
+println ""{goodbye}""

test/asm/interpolation-after-string.err

@@ -0,0 +1,14 @@
+error: interpolation-after-string.asm(5):
+    Unknown character '{'
+error: interpolation-after-string.asm(5):
+    syntax error, unexpected symbol
+while expanding symbol "greeting"
+error: interpolation-after-string.asm(5):
+    Unknown character '}'
+error: interpolation-after-string.asm(8):
+    Unknown character '{'
+error: interpolation-after-string.asm(8):
+    syntax error, unexpected symbol
+error: interpolation-after-string.asm(8):
+    Unknown character '}'
+Assembly aborted with 6 errors!