Markdown knownSchemes lacks vscode's own URI scheme

[phaseOne]

Issue Type: Bug

Issue

Create a Markdown file that contains a link with the vscode: URI scheme:

[Link to Python extension for vscode](vscode:extension/ms-python.python)

Expected link location to be vscode:extension/ms-python.python, and displayed as Link to Python extension for vscode.

Actual link location vscode-resource:/vscode:extension/ms-python.python, and displayed as Link to Python extension for vscode.

Proposed Solution

Add 'vscode:' to knownSchemes array in extensions/markdown-language-features/src/util/links.ts.

VS Code version: Code 1.32.3 (a3db5be, 2019-03-14T22:46:48.547Z)
OS version: Darwin x64 18.0.0

System Info

Item	Value
CPUs	Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz (4 x 2700)
GPU Status	2d_canvas: enabled checker_imaging: disabled_off flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on native_gpu_memory_buffers: enabled rasterization: enabled surface_synchronization: enabled_on video_decode: enabled webgl: enabled webgl2: enabled
Load (avg)	3, 3, 4
Memory (System)	8.00GB (0.02GB free)
Process Argv
Screen Reader	no
VM	0%

Extensions (34)

Extension	Author (truncated)	Version
rtf	ale	2.0.1
solargraph	cas	0.19.3
vscode-modelines	chr	2.0.2
npm-intellisense	chr	1.3.0
vscode-npm-script	eg2	0.3.5
vscode-diff	fab	1.4.0
vscode-git-history	fab	2.0.5
beautify	Hoo	1.4.11
vscode-duplicate	mrm	1.2.1
python	ms-	2019.3.6139
cpptools	ms-	0.22.1
csharp	ms-	1.18.0
mono-debug	ms-	0.15.8
incrementor	nms	0.1.0
printcode	nob	3.0.0
openhab	ope	0.4.1
vscode-docker	Pet	0.6.1
platformio-ide	pla	1.6.0
diff	raf	0.0.1
ruby	reb	0.22.3
java	red	0.41.0
LiveServer	rit	5.5.1
partial-diff	ryu	1.4.0
code-settings-sync	Sha	3.2.7
code-spell-checker	str	1.6.10
unity-debug	Uni	2.7.2
vscodeintellicode	Vis	1.1.5
vscode-arduino	vsc	0.2.25
vscode-java-debug	vsc	0.17.0
vscode-java-dependency	vsc	0.4.0
vscode-java-pack	vsc	0.6.0
vscode-java-test	vsc	0.15.1
vscode-maven	vsc	0.15.2
markdown-all-in-one	yzh	2.2.0

(1 theme extensions excluded)