Hide didact validation command except where we need it in the command palette

[bfitzpat]

Validate command doesn't need to be active for everything in the command palette. Should be able to filter it out so it only shows up with the didact webview or the editor.

We got this working for the editor:

			"commandPalette": [
				{
					"command": "vscode.didact.verifyCommands",
					"when": "resourceFilename =~ /[.](didact)[.](md|adoc)$/"
				}
			],

But we need something similar on the webview side and don't have that yet.

@apupier found this:

unless they have implemented since that time, seems a custom context needs to provided https://stackoverflow.com/questions/52142903/is-there-any-contexts-when-specified-webview-panel-is-active-in-vscode-extension