fix: prevent unsafe browser targets from issue tracker links

[ChiahongHong]

This PR prevents repository-provided issue tracker links from being passed to the Windows command shell when opening them in a browser. Shared .issuetracker rules can turn commit text into clickable links, we now validate browser targets before opening them and only allows well-formed absolute http, https, and ftp URLs.

On Windows, browser targets are now opened with UseShellExecute instead of cmd /c start, which avoids command-line parsing of link text and prevents characters in a URL from being interpreted as shell syntax. Unsafe targets are blocked with a notification instead of being opened.

I know IsWellFormedUriString is a bit picky in some cases, but given the security concerns, this approach seems reasonable, and the common URL formats I’ve tested so far should work properly.

Before

A maliciously crafted .issuetracker rule could inject shell syntax into the generated browser target:

[issuetracker "Github ISSUE"]
	regex = "#(\\d+)"
	url = https://github.com/sourcegit-scm/sourcegit/issues/$1\\\" & calc.exe

unsafe_url.mp4

After