Refactor and fix diff highlighting

[mathewpareles]

This pull request includes several commits that refactor and fix the diff highlighting feature. The changes include:

1. Creating diffs using the diffArea data structure instead of just the diff data structure.

2. Making diffs responsive on changes. Changes are handled by DisplayChangesProvider.ts, and involve creating new diffAreas and diffs, refreshing changes dynamically refreshDiffAreas, and accepting and rejecting diffs.

3. Handling diff logic entirely in the findDiffs.ts file, instead of in both getDiffedLines.js and extension.ts.

4. Renaming variables and functions. New names include acceptDiff, rejectDiff, and types have more uniform names including BaseDiffArea, DiffArea, BaseDiff, and Diff.

[mathewpareles]

Changes pass all tests.

[mathewpareles]

Note that this PR solves Issue #15.