Improve unicode string handling in linkifier

[jerch]

Had to do several changes to get the cell/length offset mess working. This PR shall fix #1669.

Changes in Buffer.ts:

• BufferStringIterator: class to get a hold of the buffer string content in a convenient way
• Buffer.stringIndexToBufferIndex: method to translate a string index from a string retrieved by Buffer.translateBufferLineToString to get the row and cols indices in the buffer. Atm this method assumes that the string started at col 0. (Can be changed easily to respect startCol as well.)
• Buffer.contents: return a BufferStringIterator

Changes in Linkifier.ts:

• reimplemented _linkifyRow and _doLinkifyRow

TODO:

• reimplement Linkifier._addLink to support unicode char offsets within a match (currently only pre/post match offsets are respected)
• tests for unicode chars in linkifier

@alexr00 The new methods in Buffer can also fix the highlight offset problems of the selection manager. Might be worth a look once this PR can land. Seems to work now. issue created #1680 + #1686

[jerch]

Ready for review 😸

[Tyriar]

Seems to work, the review is mainly clean up/strictness related stuff

[jerch]

@Tyriar I think I have addressed all review issues. Up for the next round 😸

Could you also test the issue from #1669 (comment) again? I think it is not related to the linkifier at all and will need separate tracking before closing the issue.

[Tyriar]

LGTM, I checked 3.4.0 and #1669 (comment) doesn't appear to be a regression