[CFE] Encapsulate int operations in separate classes for VM and JS

This refactoring achieves the following:

- Collect the integer operation semantics into one place instead of
having it sprinkled all over the constant evaluator with many
"if (targetingJavaScript)" tests.

- Avoid emitting the internal JavaScriptIntConstant node, which is
serialized as normal DoubleConstant nodes, thus achieving parity
between direct consumers and consumers seeing output that has been
serialized.

- Implement the JavaScript semantics for unsigned right shift.

- Fix truncating divide with a result outside int64 range incorrectly
clamping the result for JS targets.

- Fix positive hex constants >= 2^63 through int.fromEnvironment
producing negative values in dart2js.

- Clarify in Kernel documentation how numeric constants are represented
on VM vs JS targets.

Change-Id: If30bb2c2c77c54eff120b611b059c2ec726c99a0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/116525
Reviewed-by: Mayank Patke <fishythefish@google.com>