Mac/UI-consisteny: shift-continuing a double/triple-click'n drag selection does not remember its starting mode

Issue Type: <b>Bug</b>

### Summary
On the Mac, when a text selection is **initiated** by double/triple click (to get word/line-wise selection), this should be **remembered** when **continuing** this selection by shift-click and drag.

### Description
When you **double/triple** click somewhere, the clicked **word/line** is selected.
Also, when you keep the mouse down after double/triple clicking and start **dragging**, the selection starts with the clicked word/line, and as you drag along it also **grows** by words/lines (depending on number of initial clicks).

Both of these behaviours are entirely correct.

BUT, when you now release the mouse, hold down shift, and then start dragging again, VSCode forgot how the selection was started, and expands by letters only.

### To reproduce
- double click a word with the mouse to select it, OR
- double click and drag several words to select them, then
- release the mouse,
- hold down shift,
- click'n drag with the mouse somewhere outside the existing selection,

-> the selection now grows by **individual letters**, and not by **words** anymore.

Same thing for **triple** click and selecting **lines**.

### Expected behaviour
The shift-click and drag should remember if the initial selection was started with a single, double or triple click, and expand the selection accordingly. Try the same steps with _TextEdit_ or a _Mail_ Compose window or even the Github textarea input field in Safari where I'm currently entering this bug report.

VS Code version: Code 1.51.1 (e5a624b788d92b8d34d1392e4c4d9789406efe8f, 2020-11-11T01:11:34.018Z)
OS version: Darwin x64 19.6.0

<details>
<summary>System Info</summary>

|Item|Value|
|---|---|
|CPUs|Intel(R) Core(TM) i5-8500B CPU @ 3.00GHz (6 x 3000)|
|GPU Status|2d_canvas: enabled<br>flash_3d: enabled<br>flash_stage3d: enabled<br>flash_stage3d_baseline: enabled<br>gpu_compositing: enabled<br>metal: disabled_off<br>multiple_raster_threads: enabled_on<br>oop_rasterization: enabled<br>opengl: enabled_on<br>protected_video_decode: unavailable_off<br>rasterization: enabled<br>skia_renderer: disabled_off_ok<br>video_decode: enabled<br>webgl: enabled<br>webgl2: enabled|
|Load (avg)|2, 1, 1|
|Memory (System)|32.00GB (1.11GB free)|
|Process Argv|-psn_0_1061123 --crash-reporter-id 942e714c-4201-410a-9a28-266cd696ccc5|
|Screen Reader|no|
|VM|0%|
</details><details><summary>Extensions (30)</summary>

Extension|Author (truncated)|Version
---|---|---
aws-toolkit-vscode|ama|1.16.0
path-intellisense|chr|2.3.0
bracket-pair-colorizer-2|Coe|0.2.0
vscode-quick-select|dba|0.2.9
vscode-npm-script|eg2|0.3.13
auto-close-tag|for|0.5.9
auto-complete-tag|for|0.1.0
auto-rename-tag|for|0.1.5
todo-tree|Gru|0.0.188
terraform|has|2.3.0
vscode-auto-open-markdown-preview|hnw|0.0.4
vscode-power-mode|hoo|2.2.0
vscode-hacker-typer|jev|0.1.1
cypress-fixture-intellisense|Jos|1.2.0
vscode-map-preview|jum|0.5.6
hg|mrc|1.7.1
vscode-docker|ms-|1.8.1
cpptools|ms-|1.1.3
indent-rainbow|ode|7.4.0
geo-data-viewer|Ran|2.3.0
vscode-xml|red|0.14.0
preview-vscode|sea|2.2.5
code-settings-sync|Sha|3.4.3
vscode-cy-helper|She|1.0.1
ascii-plist|spe|1.0.3
code-spell-checker|str|1.10.2
html-preview-vscode|tht|0.2.5
vscodeintellicode|Vis|1.2.10
quokka-vscode|Wal|1.0.335
markdown-all-in-one|yzh|3.4.0


</details>
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