Const provides better type guards and can make logic clearer

Use `Array<T | U>` when dealing with union types instead of  `(T | U)[]`

Fixes #64633

We compile using the alwaysStrict flag so these directives are not needed

This part removes most `use strict` directives that are right after the file header

There are many places in the code that do `import Event, { ...} from '.../event'`.

* introduce IRelativePattern and use in extension API

* 💄

* docs

* introduce RelativePattern

* support RelativePattern also for file watcher

* also make findFiles support RelativePattern

* less type conversion

* add GlobPattern type and remove readonly

* make base a string

* fix setter access to RelativePattern

* fix npe when exclude is undefined

* fix findFiles: pattern seems to be matched against workspace always

* 💄

* clarify glob pattern matching