TS Modernization - Code Cleanup

More code cleanup tasks

In the effort to reduce some complexity and technical debt in the extension codebase - the following are the outstanding things to address after the TS Modernization PR is completed: #11085

• clean up clients.ActiveClient vs getActiveClient() -- pick one and stick with it.
• clean up ui vs getUI() - consistency - same thing
• elminiate use of null
• clean up use of os.platform() everywhere with the use of a constants
• unify use of guards and replace util.is* with guards
• replace filesystem wrappers (those that provide promise-based fs functions) with use of built-in node:fs.promises.XXX functions
• examine all use of new Promise(...) to ensure that there isn't a cleaner use available.
• enable plugin:@typescript-eslint/strict in eslint and fix all errors (about ~450), including:
  • A record is preferred over an index signature
  • Do not delete dynamically computed property keys
  • Expected a 'for-of' loop instead of a 'for' loop with this simple iteration
  • Expected an error object to be thrown
  • Explicit enum value must only be a literal value (string, number, boolean, etc)
  • Literals should be exposed using readonly fields
  • Prefer using an optional chain expression instead, as it is more concise and easier to read
  • Prefer using nullish coalescing operator ('??') instead of a logical or ('||'), as it is a safer operator
  • The generic type arguments should be specified as part of the constructor type arguments
  • The two values in this comparison do not have a shared enum type
  • Unexpected class with only static properties
  • Unnecessary conditional, both sides of the expression are literal values
  • Unnecessary conditional, expected left-hand side of '??' operator to be possibly null or undefined
  • Unnecessary conditional, the types have no overlap
  • Unnecessary conditional, value is always falsy
  • Unnecessary conditional, value is always truthy
  • Unnecessary optional chain on a non-nullish value
  • Use 'as XXX' instead of '<XXX>'
  • Use 'includes()' method instead
  • Use 'String#startsWith' method instead
  • Use an 'interface' instead of a 'type'
  • Use the 'String#endsWith' method instead
  • Useless constructor
  • void is not valid as a constituent in a union type
  • void is only valid as a return type or generic type argument

Refactoring:

• investigate and fix initialization and remove BlockingTask concept and move to a resettable Signal -- should make blocking at the high-order tasks much cleaner
• refactor protocolFilter to ensure order of processed messages is maintained.

Start by reviewing the completed TS Modernization PR #11085 and the outstanding cleanup areas listed here. Done requires resolving the identified TypeScript and ESLint issues, simplifying the initialization and protocolFilter designs, and addressing each consistency and promise/filesystem cleanup item.