feat(gdtk-lint): unnecessary-branch

[elenakrittik]

Implements the unnecessary-branch check which checks for else statements that are preceded by ifs/elifs that always return.

Blocked on #53's item #4

[elenakrittik]

The diagnostic for this is, mildly saying, subpar. However, it cannot be improved in the scope of this PR (requires at least making Diagnostic/Highlight own .span, and ideally "just" improving the output), and given that it's functional i'm inclined to merge it.

Also, all the BranchGroup logic sounds like something a future semantic analyzer should do, but, again, it's out-of-scope for this PR.