Default `attachStacktrace` to `true` for `captureMessage`

[nicohrubec]

Eventually captureMessage will be unified with captureException. As a pre-step, we should align behavior between these APIs by defaulting attachStacktrace to true for captureMessage.

Currently captureException always includes a stack trace, while captureMessage only does so when attachStacktrace is explicitly enabled (it defaults to false). Switching this default makes captureMessage behave consistently with captureException, so that unifying them later becomes straightforward.

[linear-code]

JS-2618