Use changelog bullet character that's already in use on other items.

[vaind]

Markdown in changelog.md allows users to use either * or - for an unordered (bullet) list. Currently, the code that updates the changelog always adds new items with -.

Instead, the script should determine what separator is in use and use that instead so that the changelog keeps looking good in its raw source form.

[marandaneto]

One of the reasons is that pub.dev has a default pattern for changelog items.