move _data from BufferLine to new LogicalLine class

[PerBothner]

This moves cell data from a BufferLine into a new LogicalLine class which contains the actual data for a line. This data does not change if Buffer width changes. Each LogicalLine is rendered as one or more BufferLines, depending on terminal width; conversely each BufferLine specifies a sub-range of the parent LogicalLine.

This implements issue #5673 .

Some benefits:

• Simpler and faster Buffer.resize.
  No need for cleaning up memory after resize.
• Potentially handle reflow lazily, only reflowing visible lines.
• Faster getBlankLine.
• Do not need memory for empty cells at end of lines.
• Better conceptual model/view separation: The LogicalLine is the "model" (conceptual buffer data) while the set of BufferLines is the "view" (how the LogicalView is rendered given a specifical line width).
• Potentially support multiple "views" of the same model at the same time. For example people might be collaboratively viewing the same terminal, but with different line widths.
• End-of-line background color persists over resize.

Memory cleanup: There is no need for queing up memory-cleanup items when doing a resize, since resize doesn't create garbage (except when lines are trimmed). However, it may be worthwhile doing memory cleanup when lines are erased and/or when lines are unwrapped (commonly done together). We might also cleanup memory when lines move out above ybase.

Merging BufferReflow.ts into Buffer.ts: An optional follow-up would be to merge the rest of BufferReflow.ts into Buffer.ts - since there isn't much left in BufferReflow.ts. See this changeset. That would make more slightly more efficient and (in my opinion) more readable code, but I didn't do this in the interest of a more manageable PR.

[PerBothner]

Note this PR assumes PR #5788 has been merged first.

[PerBothner]

FWIW, I'm looking into adding addons-search support to the DomTerm port, and it appears using LogicalLine can somewhat simplify the implementation of translateBufferLineToStringWithWrap.

(A possible future improvement I'm thinking about is to store the actual values of translateToString in the LogicalLine, replacing the _combined map. This would eliminate the need for the SearchLineMap.)

[PerBothner]

I merged from upstream/master (with some effort thanks to the new string aching). However, the image addon breaks the BuferLine abstraction. I'll take a look at that shortly. I may focus on fixing that in the context PR #5853, which I haven't merged yet. (I would also like to further simplify the reflow logic, which I hope allow me fix the remaining testsuite failures.)