[devkit/utilities] Implement fee formatting functions

Format fee values for display in different contexts.

**Functions:**
```rust
pub fn format_stroops(n: u64) -> String  // "3,849 str"
pub fn format_xlm(n: u64) -> String      // "0.0003849 XLM"
pub fn format_fee_short(n: u64) -> String // auto: str below 1M, XLM above
```

**Files to touch:**
- `packages/devkit/src/utilities/formatters.rs`