Fix #2059 turf-boolean-within returning false positive when line ends outside poly

[kudlav]

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In boolean-within check if the LineString ends outside of the Polygon.
This PR fixes issue #2059.

Tested by the following two test cases:

// should return false
const resRobertOrthofer = turf.booleanWithin({
  "type": "LineString",
  "coordinates": [
    [ -26.19140625, 46.800059446787316 ],
    [ -38.14453125, 45.82879925192134 ],
    [-35.15625, 39.90973623453719]
  ]
}, {
  "type": "Polygon",
  "coordinates": [[
    [ -54.84375, 43.70759350405294 ],
    [ -55.1953125, 29.53522956294847 ],
    [ -39.375, 30.14512718337613 ],
    [ -43.59375, 41.11246878918088 ],
    [ -25.48828125, 43.32517767999296 ],
    [ -21.796875, 34.30714385628804 ],
    [ -15.468749999999998, 36.59788913307022 ],
    [ -19.16015625, 49.26780455063753 ],
    [ -54.84375, 43.70759350405294 ]
  ]]
});

// should return false
const resKudlav = turf.booleanWithin({
  "type": "LineString",
  "coordinates": [
    [9.15, 32.46],
    [12.33, 41.6]
  ]
}, {
  "type": "Polygon",
  "coordinates": [[
    [ -0.64, 23.73 ],
    [ 2.22, 24.89 ],
    [ 7.13, 31.6 ],
    [ 14.9, 33.93 ],
    [ 23.41, 32.26 ],
    [ 23.99, 34.46 ],
    [ 27.12, 34.89 ],
    [ 28.56, 38.13 ],
    [ 33.68, 41.6 ],
    [ 13.6, 41.36 ],
    [ 2.53, 38.38 ],
    [ -0.3, 36.01 ],
    [ -0.64, 23.73 ]
  ]]
});

[twelch]

Thank you for your contribution @kudlav! This looks correct in that it now ensures to check the last point in the linestring, and then is sure to only calculate midpoint after if it's not the last point. Adding a test case to the PR to demonstrate is helpful, but I don't see reason to hold this up.

A new v7 prerelease should be published to npm after merge if you'd like to verify the fix.