Allow escaped " literals in the SyntaxScopeMap

In order to pass a quote literal to the postcss-selector-parser, it 
needs to be escaped. However, this escaping is not removed by the parser 
in the yielded value token, so in this commit I replace any escaped 
quote literals with unescaped quotes after we trim the outer quotes off 
of the string.

spec/syntax-scope-map-spec.js

@@ -49,13 +49,15 @@ describe('SyntaxScopeMap', () => {
     const map = new SyntaxScopeMap({
       '"b"': 'w',
       'a > "b"': 'x',
-      'a > "b":nth-child(1)': 'y'
+      'a > "b":nth-child(1)': 'y',
+      '"\\""': 'z'
     })
 
     expect(map.get(['b'], [0], true)).toBe(undefined)
     expect(map.get(['b'], [0], false)).toBe('w')
     expect(map.get(['a', 'b'], [0, 0], false)).toBe('x')
     expect(map.get(['a', 'b'], [0, 1], false)).toBe('y')
+    expect(map.get(['a', '"'], [0, 1], false)).toBe('z')
   })
 
   it('supports the wildcard selector', () => {

src/syntax-scope-map.js

@@ -36,7 +36,7 @@ class SyntaxScopeMap {
 
             case 'string':
               if (!currentTable) currentTable = this.anonymousScopeTable
-              const value = termNode.value.slice(1, -1)
+              const value = termNode.value.slice(1, -1).replace(/\\"/g, '"')
               if (!currentTable[value]) currentTable[value] = {}
               currentTable = currentTable[value]
               if (currentIndexValue != null) {