Fix crash in Pattern Language syntax highlighting for self-inheriting structs (#2546)

``` rust
struct Rec : Rec {
};
```

Prevent infinite recursion in appendInheritances by erasing processed
inheritance entries during traversal, and safely iterate over
m_inheritances in appendInheritances to avoid
modification-during-iteration issues.

Co-authored-by: paxcut <53811119+paxcut@users.noreply.github.com>