Improve JsString performance (#2042)

It changes the following:

- The current `JsString` implementation has a lot of duplicate heap allocations. For static strings, `JsString` only needs to store the index of `CONSTANTS_ARRAY`. ~~I let `JsString` can store pointer to heap or static area. The two are distinguished by the first bit. (This implementation is rough, maybe we should put flag into `Inner`, like [arcstr](https://github.com/thomcc/arcstr/blob/70ba2fac19d3efe8d2e0231daaf74f9987c04b8a/src/arc_str.rs#L751-L757))~~
- I also added more string constants, which are always used.

boa_engine/src/bigint.rs

@@ -180,7 +180,7 @@ impl JsBigInt {
             let inner = if n > 0 {
                 x.inner.as_ref().clone().shr(n as usize)
             } else {
-                x.inner.as_ref().clone().shl(n.abs() as usize)
+                x.inner.as_ref().clone().shl(n.unsigned_abs())
             };
 
             Ok(Self::new(inner))
@@ -195,7 +195,7 @@ impl JsBigInt {
             let inner = if n > 0 {
                 x.inner.as_ref().clone().shl(n as usize)
             } else {
-                x.inner.as_ref().clone().shr(n.abs() as usize)
+                x.inner.as_ref().clone().shr(n.unsigned_abs())
             };
 
             Ok(Self::new(inner))