This adds a new `DiffUnified()` method which takes a `[]Diff` slice and formats
it in the "unified diff" format. The `[]Diff` slice does not necessarily need
to be a linewise diff.

The implementation deals gracefully with missing newlines at the end of either
side, as well as empty inputs on either side.

The method accepts two optional arguments: `UnifiedLabels()` for setting the
labels for the two inputs and `UnifiedContextLines()` for setting the number of
context lines printed.

Example usage:

```go
unified := dmp.DiffUnified(diffs,
	diffmatchpatch.UnifiedLabels("text1", "text2"),
	diffmatchpatch.UnifiedContextLines(3))
```

A convenience method, `Unified()`, takes two string inputs and returns the
unified diff.

Fixes: #124

Re-use the existing `Patch` struct to store "hunks" instead of adding a new
struct for this purpose.

- Add DiffLineMode method for always-on line mode diffing
- Introduce DiffFunction type for flexible diff computation
- Add DiffCleanupLineBased for line-specific diff optimization
- Refactor diffMainRunes to use closure-based approach
- Update Go version to 1.24 and clean up dependencies
- Add comprehensive test coverage for new functionality

- Update Go version from 1.13 to 1.24 in go.mod
- Add mise.toml configuration for development tools setup
- Include tasks for installing Go development tools (gopls, golangci-lint, delve)

- Update testify from v1.4.0 to v1.10.0
- Update yaml from v2.4.0 to v3.0.1
- Add go-difflib v1.0.0 as indirect dependency
- Clean up go.sum checksums