fix: make static data variable threadsafe

I've hit a bug developing a go parser and running multiple instances of
it with t.Parallel()

Turned out that staticData var was a global and would throw
concurrent data access errors when running the test suite
with -race

This commit fixes it by doing the following:

1. turn <parser.grammarName; format="cap">ParserStaticData into a struct
2. Teach <parser.name; format="cap">Init() to return a pointer to new instance of this struct
3. Teach <parser.name; format="cap">Init() to initialize it upon construction

I've been able to run 10000 concurrent parsers in this setup without
having any performance/memory usage problem

Signed-off-by: Tomasz Nguyen <me@swistofon.pl>
Signed-off-by: Tomasz Nguyen <tonguyen@confluent.io>

tool/resources/org/antlr/v4/tool/templates/codegen/Go/Go.stg

@@ -166,7 +166,7 @@ type <parser.name> struct {
 	<endif>
 }
 
-var <parser.grammarName; format="cap">ParserStaticData struct {
+type <parser.grammarName; format="cap">ParserStaticData struct {
   once                   sync.Once
   serializedATN          []int32
   LiteralNames           []string
@@ -198,10 +198,9 @@ func <parser.grammarName; format="lower">ParserInit() {
   staticData.serializedATN = <atn>
   deserializer := antlr.NewATNDeserializer(nil)
   staticData.atn = deserializer.Deserialize(staticData.serializedATN)
-  atn := staticData.atn
-  staticData.decisionToDFA = make([]*antlr.DFA, len(atn.DecisionToState))
+  staticData.decisionToDFA = make([]*antlr.DFA, len(staticData.atn.DecisionToState))
   decisionToDFA := staticData.decisionToDFA
-  for index, state := range atn.DecisionToState {
+  for index, state := range staticData.atn.DecisionToState {
     decisionToDFA[index] = antlr.NewDFA(state, index)
   }
 }
@@ -210,14 +209,15 @@ func <parser.grammarName; format="lower">ParserInit() {
 // static state used to implement the parser is lazily initialized during the first call to
 // New<parser.name>(). You can call this function if you wish to initialize the static state ahead
 // of time.
-func <parser.name; format="cap">Init() {
-  staticData := &<parser.grammarName; format="cap">ParserStaticData
-  staticData.once.Do(<parser.grammarName; format="lower">ParserInit)
+func <parser.name; format="cap">Init() *<parser.grammarName; format="cap">ParserStaticData {
+  staticData := &<parser.grammarName; format="cap">ParserStaticData{}
+  <parser.grammarName; format="lower">ParserInit(staticData)
+  return staticData
 }
 
 // New<parser.name> produces a new parser instance for the optional input antlr.TokenStream.
 func New<parser.name>(input antlr.TokenStream) *<parser.name> {
-	<parser.name; format="cap">Init()
+	staticData := <parser.name; format="cap">Init()
 	this := new(<parser.name>)
 	this.BaseParser = antlr.NewBaseParser(input)
   staticData := &<parser.grammarName; format="cap">ParserStaticData