fix(es/parser): Disallow NumericLiteralSeparator with BigInts (#11510)

**Description:**

I'm using swc in my JS Engine
[Yavashark](github.com/Sharktheone/yavashark). I noticed, some test262
tests that were crashing in swc internally. Example:
[test262](https://github.com/tc39/test262/blob/main/test/language/literals/bigint/numeric-separators/numeric-separator-literal-bil-bd-nsl-bd-err.js),
[result](https://yavashark.dev/test262/#/v/language/literals/bigint/numeric-separators/numeric-separator-literal-bil-bd-nsl-bd-err.js).

These tests are panicking because there was a missing check whether the
literal contains any separators / underscores which would cause a
failure in the `num_bigint` parser.

I've also added a fallback to not panic even when the `num_bigint` parse
fails.

Author: Sharktheone

.changeset/pink-goats-learn.md

@@ -0,0 +1,6 @@
+---
+swc_ecma_parser: patch
+swc_core: patch
+---
+
+fix(es/parser): Disallow NumericLiteralSeparator with BigInts

crates/swc_ecma_parser/src/lexer/mod.rs

@@ -1160,7 +1160,21 @@ impl<'a> Lexer<'a> {
             self.input_slice_str(lazy_integer.start, self.cur_pos())
         };
         if self.eat(b'n') {
-            let bigint_value = num_bigint::BigInt::parse_bytes(s.as_bytes(), RADIX as _).unwrap();
+            if s.starts_with('_') {
+                return Err(Error::new(
+                    Span::new(lazy_integer.start, lazy_integer.end),
+                    SyntaxError::NumericSeparatorIsAllowedOnlyBetweenTwoDigits,
+                ));
+            }
+
+            let Some(bigint_value) = num_bigint::BigInt::parse_bytes(s.as_bytes(), RADIX as _)
+            else {
+                // just a fallback in case there is anything we did not catch
+                return Err(Error::new(
+                    Span::new(lazy_integer.start, lazy_integer.end),
+                    SyntaxError::ExpectedDigit { radix: RADIX },
+                ));
+            };
             return Ok(Either::Right(Box::new(bigint_value)));
         }
         let s = remove_underscore(s, has_underscore);