(MAINT) Add Vale config to repo (#9937)

Prior to this change, using Vale to lint the docs
required knowledge of the tool and where to
retrieve the configuration and rules.

This change adds a new `.vale.ini` file to the
repository root and configures it to use our own
published prose style package.

It also ensures the `.vale` and `.vscode/styles`
folders are ignored by git for contributors who
use `Install-WorkspaceVale` and sync the style
packages respectively.

Author: michaeltlombardi

.gitignore

@@ -17,3 +17,5 @@ updatablehelp/
 xhtml/
 **/settings.json
 StaleContentReport.*.csv
+.vscode/styles
+.vale/

.vale.ini

@@ -0,0 +1,10 @@
+StylesPath    = .vscode/styles
+Packages      = https://microsoft.github.io/Documentarian/packages/vale/PowerShell-Docs.zip
+MinAlertLevel = suggestion
+
+[*]
+BasedOnStyles = PowerShell-Docs
+Vale.Spelling = NO              # We use cspell
+
+[*.md]
+BasedOnStyles = PowerShell-Docs