Support SystemVerilog DPI and DPI-C import statements

Ref: Section "35.5.4 Import declarations" from SystemVerilog standard
IEEE 1800-2017.

lexers/s/systemverilog.go

@@ -18,6 +18,7 @@ var Systemverilog = internal.Register(MustNewLexer(
 		"root": {
 			{"^\\s*`define", CommentPreproc, Push("macro")},
 			{`^(\s*)(package)(\s+)`, ByGroups(Text, KeywordNamespace, Text), nil},
+			{`^(\s*)(import)(\s+)("DPI(?:-C)?")(\s+)`, ByGroups(Text, KeywordNamespace, Text, LiteralString, Text), nil},
 			{`^(\s*)(import)(\s+)`, ByGroups(Text, KeywordNamespace, Text), Push("import")},
 			{`\n`, Text, nil},
 			{`\s+`, Text, nil},

lexers/testdata/systemverilog.actual

@@ -0,0 +1,11 @@
+// Comment
+
+program top;
+
+  import "DPI-C" hello=task hello();
+
+  initial begin
+    hello();
+  end
+
+endprogram : top

lexers/testdata/systemverilog.expected

@@ -0,0 +1,35 @@
+[
+  {"type":"CommentSingle","value":"// Comment\n"},
+  {"type":"Text","value":"\n"},
+  {"type":"Keyword","value":"program"},
+  {"type":"Text","value":" "},
+  {"type":"Name","value":"top"},
+  {"type":"Punctuation","value":";"},
+  {"type":"Text","value":"\n\n  "},
+  {"type":"KeywordNamespace","value":"import"},
+  {"type":"Text","value":" "},
+  {"type":"LiteralString","value":"\"DPI-C\""},
+  {"type":"Text","value":" "},
+  {"type":"Name","value":"hello"},
+  {"type":"Operator","value":"="},
+  {"type":"Keyword","value":"task"},
+  {"type":"Text","value":" "},
+  {"type":"Name","value":"hello"},
+  {"type":"Punctuation","value":"();"},
+  {"type":"Text","value":"\n\n  "},
+  {"type":"Keyword","value":"initial"},
+  {"type":"Text","value":" "},
+  {"type":"Keyword","value":"begin"},
+  {"type":"Text","value":"\n    "},
+  {"type":"Name","value":"hello"},
+  {"type":"Punctuation","value":"();"},
+  {"type":"Text","value":"\n  "},
+  {"type":"Keyword","value":"end"},
+  {"type":"Text","value":"\n\n"},
+  {"type":"Keyword","value":"endprogram"},
+  {"type":"Text","value":" "},
+  {"type":"Operator","value":":"},
+  {"type":"Text","value":" "},
+  {"type":"Name","value":"top"},
+  {"type":"Text","value":"\n"}
+]