Support ‘is’ keyword

This change makes Splash support the `is` keyword, for example in type
pattern matching in `switch` statements.

Sources/Splash/Grammar/SwiftGrammar.swift

@@ -48,7 +48,7 @@ private extension SwiftGrammar {
         "#selector", "required", "willSet", "didSet",
         "lazy", "subscript", "defer", "inout", "while",
         "continue", "fallthrough", "repeat", "indirect",
-        "deinit"
+        "deinit", "is"
     ] as Set<String>).union(accessControlKeywords)
 
     static let accessControlKeywords: Set<String> = [

Tests/SplashTests/Tests/StatementTests.swift

@@ -169,6 +169,39 @@ final class StatementTests: SyntaxHighlighterTestCase {
         ])
     }
 
+    func testSwitchStatementWithTypePatternMatching() {
+        let components = highlighter.highlight("""
+        switch variable {
+        case is MyType: break
+        default: break
+        }
+        """)
+
+        XCTAssertEqual(components, [
+            .token("switch", .keyword),
+            .whitespace(" "),
+            .plainText("variable"),
+            .whitespace(" "),
+            .plainText("{"),
+            .whitespace("\n"),
+            .token("case", .keyword),
+            .whitespace(" "),
+            .token("is", .keyword),
+            .whitespace(" "),
+            .token("MyType", .type),
+            .plainText(":"),
+            .whitespace(" "),
+            .token("break", .keyword),
+            .whitespace("\n"),
+            .token("default", .keyword),
+            .plainText(":"),
+            .whitespace(" "),
+            .token("break", .keyword),
+            .whitespace("\n"),
+            .plainText("}")
+        ])
+    }
+
     func testForStatementWithStaticProperty() {
         let components = highlighter.highlight("for value in Enum.allCases { }")
 
@@ -272,6 +305,7 @@ extension StatementTests {
             ("testSwitchStatement", testSwitchStatement),
             ("testSwitchStatementWithAssociatedValues", testSwitchStatementWithAssociatedValues),
             ("testSwitchStatementWithFallthrough", testSwitchStatementWithFallthrough),
+            ("testSwitchStatementWithTypePatternMatching", testSwitchStatementWithTypePatternMatching),
             ("testForStatementWithStaticProperty", testForStatementWithStaticProperty),
             ("testForStatementWithContinue", testForStatementWithContinue),
             ("testRepeatWhileStatement", testRepeatWhileStatement)