Implement proper line split + tests - #1544

Fixes #1543

• Changes were applied incorrectly. Changes across line spans caused buffer corruption. Typical example is undoing multiple changes via fast Ctrl+Z.
• Simplified change handling: per LSP, any sequence applies

 * The actual content changes. The content changes describe single state changes
 * to the document. So if there are two content changes c1 and c2 for a document
 * in state S then c1 move the document to S' and c2 to S''.

and we can now handle this since line splits update after each change.

• Implement new line location without creating extra strings
• Add tests for new lines as well as for multi-line edits.
• Remove unused flags
• Remove change reordering/versions.