Introduce readonly operator

[chshanovskiy]

Description

See #311 issue.

Checklist for a new method

• Create a directory for the new method in the src directory in param-case
• Place the source code to src/method-name/index.ts in ESModules export in camelCase named export
• Add tests to src/method-name/method-name.test.ts
• Add fork tests to src/method-name/method-name.fork.test.ts
• Add type tests to test-typings/method-name.ts
  • Use // @ts-expect-error to mark expected type error
  • import { expectType } from 'tsd' to check expected return type
• Add documentation in src/method-name/readme.md
  • Add header Patronum/MethodName
  • Add section with overloads, if have
  • Add Motivation, Formulae, Arguments and Return sections for each overload
  • Add useful examples in Example section for each overload
• Add section to README.md in the repository root
  • Add method to the table of contents into correct category - [MethodName](#methodname) - description.
  • Add section ## MethodName
  • Add [Method documentation & API](/src/method-name) into section
  • Add simple example

[zerobias]

Is it necessary to do .map call when unit is already derived? We have is. targetable to detect this case and just return unit as it is

[chshanovskiy]

@zerobias , I've added tests and implementation for derived units in argument