feat(tools-filesystem): Add a richer set of filesystem helpers to tools-filesystem

[JasonVMo]

Description

This enhances the set of filesystem helpers in tools-filesystem.

Changes to existing functions

• renamed ensureDir and ensureDirForFile to follow standard sync naming conventions.
• restructured the files to create an index file and dedicated files for types of functionality.
• enhanced the reworked the mockFS to provide much more functionality including promise based mocks.

New functionality

• asynchronous versions of the helper functions. Pairings follow fs naming (e.g. ensureDirSync + ensureDir)
• helpers for reading files with UTF encoding
• helpers for parsing JSON with BOM stripping, and serializing with trailing "\n" addition which is always omitted by JSON.serialize.
• helpers for reading/writing JSON files which use these helpers.
• optimized file compares which do cheap compares (size, ino/dev) then content comparisons. Sync and async versions provided.
• FSEntry class which provides a wrapper around a path, progressively caching information and coalescing promise based filesystem calls. This is useful to either implement a caching layer, or to be handed out by a caching layer.
• ability to use the mockFS to test the new functionality.
• tests for all the new functionality.

Test plan

Automated tests.