Add maximized mode to markdown-textarea + CSS improvements (#2660)

Add maximized mode to markdown-textarea + CSS improvements (#2660)

* Add arrows-angle-contract/expand bootstrap icons

* Add grey textarea-background-color

* Add maximized support to markdown-textarea + improve column display

* Refactor CSS + add ResizeObservable

* Replace bootstrap icons with softies

* Add ResizeObserver typing definition

* Add focus on textarea + Fix Observables

* Propage component changes on markdown plugins

* Ignore ResizeObserver not implemented in typescript yet

* Move observers from constructor to click event

* Add scss and css variables

* Replace textareaWidth with textareaMaxWidth to fix others textareas

* Clean unused css rules

* Fix ResizeObserver unknown by TypeScript compiler

* Set max-width: 100% for small and mobile views

* Fix textarea/preview height on maximized mode

* Add common padding textarea/preview side-by-side

* Hide scrollbar sub-menu on small-views

* Add maximized mode for mobile views

* Fix sass calculate syntax

* Revert custom CSS variable for inputBorderRadius and inputBorderColor

* Remove unsued methods

* Fix missing implement method

Co-authored-by: kimsible <kimsible@users.noreply.github.com>