Add RegExp.flags external and corresponding tests

Author: jmagaram

runtime/Stdlib_RegExp.res

@@ -22,5 +22,6 @@ module Result = {
 @get external source: t => string = "source"
 @get external sticky: t => bool = "sticky"
 @get external unicode: t => bool = "unicode"
+@get external flags: t => string = "flags"
 
 external ignore: t => unit = "%ignore"

runtime/Stdlib_RegExp.resi

@@ -298,10 +298,25 @@ Console.log(regexp2->RegExp.unicode) // Logs `true`, since `u` is set
 @get
 external unicode: t => bool = "unicode"
 
+/**
+`flags(regexp)` returns a string consisting of all the flags set on this `RegExp`.
+
+See [`RegExp.flags`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/flags) on MDN.
+
+## Examples
+```rescript
+let regexp = RegExp.fromString("\\w+", ~flags="gi")
+Console.log(regexp->RegExp.flags) // Logs "gi", all the flags set on the RegExp
+```
+*/
+@get
+external flags: t => string = "flags"
+
 /**
   `ignore(regExp)` ignores the provided regExp and returns unit.
 
   This helper is useful when you want to discard a value (for example, the result of an operation with side effects)
   without having to store or process it further.
 */
 external ignore: t => unit = "%ignore"
+//

tests/tests/src/core/Core_RegExpTest.res

@@ -0,0 +1,18 @@
+// Test for RegExp.flags
+Test.run(
+  __POS_OF__("RegExp.flags basic"),
+  RegExp.fromStringWithFlags("\\w+", ~flags="gi")->RegExp.flags,
+  eq,
+  "gi",
+)
+
+// Test for alphabetical sorting of flags
+Test.run(
+  __POS_OF__("RegExp.flags sorting"),
+  RegExp.fromStringWithFlags("\\w+", ~flags="igd")->RegExp.flags,
+  eq,
+  "dgi",
+)
+
+// Test with no flags
+Test.run(__POS_OF__("RegExp.flags empty"), RegExp.fromString("\\w+")->RegExp.flags, eq, "")

tests/tests/src/core/Core_TestSuite.res

@@ -11,3 +11,4 @@ include Core_JsonTests
 include Core_NullableTests
 include Core_DictTests
 include Core_IteratorTests
+include Core_RegExpTest