Scans all yaml files with cfn-lint at startup.

Issue Type: Bug

I have a couple of hundred cloudformation templates and serverless templates in my default workspace.

Every time I start or reload vscode it scans all the yaml files with cfn-lint.

It pushes to load average up to approx 200, until the WSL stops responding and disconnects.
Then reconnecting it does it again. Rinse, repeat.

I've had to start running
```
pkill -f cfn-lint
```
multiple times in a terminal session when vscode or my machine reloads to keep it under control.

Is there a way to add a setting, to only lint files on open, rather than scan the whole workspace on load?

Extension version: 0.20.0
VS Code version: Code - Insiders 1.61.0-insider (aec6ee09fa1021cb59c7f4f4d000b027febc967d, 2021-09-03T05:14:10.486Z)
OS version: Windows_NT x64 10.0.22000
Restricted Mode: No
Remote OS version: Linux x64 5.10.43.3-microsoft-standard-WSL2

System Info

|Item|Value|
|---|---|
|CPUs|AMD Ryzen 9 3900X 12-Core Processor (24 x 3793)|
|GPU Status|2d_canvas: enabled
gpu_compositing: enabled
multiple_raster_threads: enabled_on
oop_rasterization: enabled
opengl: enabled_on
rasterization: enabled
skia_renderer: enabled_on
video_decode: enabled
vulkan: disabled_off
webgl: enabled
webgl2: enabled|
|Load (avg)|undefined|
|Memory (System)|63.94GB (40.65GB free)|
|Process Argv|--crash-reporter-id d84130c3-89a0-47ae-a4ce-aa1e8a9d5382|
|Screen Reader|no|
|VM|0%|

|Item|Value|
|---|---|
|Remote|WSL: Ubuntu-20.04|
|OS|Linux x64 5.10.43.3-microsoft-standard-WSL2|
|CPUs|AMD Ryzen 9 3900X 12-Core Processor (24 x 3792)|
|Memory (System)|31.33GB (28.20GB free)|
|VM|0%|

A/B Experiments

```
vsliv695:30137379
vsins829:30139715
vsliv368cf:30146710
vsreu685:30147344
python383cf:30185419
vspor879:30202332
vspor708:30202333
vspor363:30204092
pythonvspyt639:30291487
pythontb:30258533
vshan820:30294714
pythonvsuse255:30319630
3f5j7703:30294757
vscod805cf:30301675
vsccppwtct:30312693
bridge0708:30335490
pygetstartedt2:30353727
dockerwalkthru:30348713
bridge0723:30353136
javagetstartedt:30350119
pydsgst2:30361790

```

Start by tracing the extension's startup behavior that launches cfn-lint across workspace YAML files, then identify how open-file linting is currently triggered. Done means a setting or behavior prevents whole-workspace scans while preserving linting for files opened by the user; verify it against a workspace containing many CloudFormation templates.

Written by the indexing model from the issue text.