DrawLineString improvements

[supersonicclay]

This PR does the following:

• DrawLineStringMode is now only used for drawing a new LineString feature. User can double-click to finish drawing the line.
• A new ExtendLineStringMode is used to extend an existing LineString feature.

[coveralls]

Pull Request Test Coverage Report for Build 1826

• 17 of 55 (30.91%) changed or added relevant lines in 3 files are covered.
• 548 unchanged lines in 15 files lost coverage.
• Overall coverage decreased (-13.5%) to 22.011%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
modules/edit-modes/src/lib/draw-line-string-mode.js	13	22	59.09%
modules/edit-modes/src/lib/extend-line-string-mode.js	0	29	0.0%

Files with Coverage Reduction	New Missed Lines	%
modules/edit-modes/src/lib/draw-line-string-mode.js	2	55.56%
modules/layers/src/mode-handlers/duplicate-handler.js	3	0%
modules/layers/src/mode-handlers/draw-rectangle-handler.js	8	0%
modules/layers/src/mode-handlers/two-click-polygon-handler.js	9	0%
modules/layers/src/mode-handlers/elevation-handler.js	19	0%
modules/layers/src/mode-handlers/translate-handler.js	26	0%
modules/layers/src/mode-handlers/rotate-handler.js	27	0%
modules/layers/src/mode-handlers/scale-handler.js	27	0%
modules/layers/src/mode-handlers/draw-90degree-polygon-handler.js	29	0%
modules/layers/src/mode-handlers/draw-line-string-handler.js	43	0%

Totals
Change from base Build 1822:	-13.5%
Covered Lines:	867
Relevant Lines:	3776

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💛 - Coveralls