Expand selection in words with underscores is inconsistent

[artem-karpenko]

Issue Type: Bug

1. Open any file which contains word consisting of letters with underscores (f.e. hello_world). For simplicity let's assume only one underscore is present.
2. Place cursor in various places of this word and activate expand selection command (either via keyboard shortcut or keybinding)
3. Observe:
   3a. cursor is not near underscore (underscore is not immediately to the left or right of cursor) - part of the word selected, bounded by word beginning/end and underscore - OK
   3b. underscore is immediately left to the cursor - part of word to the right of cursor is selected - OK
   3c. underscore is immediately right to the cursor - the whole word is selected - UNEXPECTED.

For 3c I expected only part of the word selected as well, not the whole word.

VS Code version: Code 1.42.0 (ae08d54, 2020-02-06T10:51:34.058Z)
OS version: Windows_NT x64 10.0.18362

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz (8 x 2112)
GPU Status	2d_canvas: enabled flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: disabled_software metal: disabled_off multiple_raster_threads: enabled_on oop_rasterization: disabled_off protected_video_decode: enabled rasterization: enabled skia_renderer: disabled_off surface_control: disabled_off surface_synchronization: enabled_on video_decode: enabled viz_display_compositor: enabled_on viz_hit_test_surface_layer: disabled_off webgl: enabled_readback webgl2: enabled_readback
Load (avg)	undefined
Memory (System)	15.75GB (4.81GB free)
Process Argv
Screen Reader	no
VM	0%

Extensions (10)

Extension	Author (truncated)	Version
quick-open-prefill-selection	col	0.0.1
vscode-eslint	dba	2.0.15
gitlens	eam	10.2.1
EditorConfig	Edi	0.14.4
vscode-great-icons	emm	2.1.47
intellij-idea-keybindings	k--	0.2.37
camelcasenavigation	map	1.1.1
material-icon-theme	PKi	3.9.3
partial-diff	ryu	1.4.1
terminal-tabs	Tyr	0.2.0

(5 theme extensions excluded)

[vhpgomes]

Tried to fix this in the PR #90736

PS - this is my first PR for a OSS, ever... please forgive me for any mistake or misconduct.