fix(language-server): correctly calculate coalesced document change

[rchl]

fixes vuejs/language-tools#4909

calculateChangeRange was incorrectly calculating combined change if the document update contained multiple changes. It calculated offset for each change using the original state of the document as a base while the offsets for the next change need to be calculated based on a document state with previous change applied.

I'm guessing that vscode coalesced changes so generally there was just a single one and the issue was never apparent there but with other editors there is no such guarantee.

To fix I've dropped the incorrect code and instead switched to converting changes to ts.TextChangeRanges and using existing combineChangeRanges utility function that seems to be doing the right thing.

Calculating incorrect range for the snapshot resulted in incorrectly applying changes to the SFC, causing the generated document to end up with pretty much random content and causing random issues with different features.

[johnsoncodehk]

Your method is simpler, thank you!