Readonly helps with type guards and code comprehension

This allows to declare more services declaratively.

Enables strict null checking for `textEditorModel`. I also had to add a few null supressions to difficult errors.

Allow to register a tokenizer promise and honor tokenizer promises when dealing with embedded modes in colorizing API

* new reg service and always on mnemonics

* missed one

* use new node module and accessibility service

* add typings

* adopt accessibility service

* make it lazy

**Problem**
`ITheme.getColor` is one of the major users of the type `null` in our codebase. However this function is used in many places that expected an `undefined` type instead. With the strict null work, this means that we have to write code such as `theme.getColor(...) || undefined` in some cases or have to work with  `Color | null | undefined` types.

**Fix**
`undefined` is generally better supported by TypeScript and more natural in JavaScript. This change makes `ITheme.getColor` return `Color | undefined` instead of `Color | null`.

Fix #67011

* GoToLine: Screenreader output line number, handle #66245, #65219

* Tweak value checking, logic

* example WIP 1

* Add shortcut for example 4 - WIP spaghetti

* Add examples in GoToAction2 WIP

* Rewind to  Example 1 PR

* Address @bpasero comments

* Mirror changes in standalone editor

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