Move finding toc.yml files logic to async method in Books (#6623)

* moved findTOC logic to async function

* set max number of subdirectories to search

* log error, change for loop syntax

* hadnling edge cases

extensions/notebook/package.json

@@ -17,6 +17,11 @@
 			"type": "object",
 			"title": "%notebook.configuration.title%",
 			"properties": {
+				"notebook.maxBookSearchDepth": {
+					"type": "number",
+					"default": 5,
+					"description": "%notebook.maxBookSearchDepth.description%"
+				},
 				"notebook.pythonPath": {
 					"type": "string",
 					"default": "",
@@ -350,6 +355,7 @@
 		"@types/rimraf": "^2.0.2",
 		"decompress": "^4.2.0",
 		"error-ex": "^1.3.1",
+		"fast-glob": "^3.0.4",
 		"figures": "^2.0.0",
 		"fs-extra": "^5.0.0",
 		"glob": "^7.1.1",