dayjs 2.0 - add tests from dayjs1.0 and fix findings

[BePo65]

Working on the utc plugin for dayjs 2.0 I added more tests from dayjs to get a clean base for further development. Here a list of additions / changes:

• build: update packages
• docs: update 'TODO' part in README.md
• refactor: change name of parameter 'number' to 'value' in add / subtract
• refactor: move type declaration 'GetterFn' to types.ts
• fix: add 'ms' to parameter type of 'unit' of 'diff'
• test: add missing tests for locales
• test: add test 'display' and tests for required locales
• fix: utcOffset, when offset has minutes <> '0' or '30'
• refactor: derive 'UnitBase' from 'UnitLong'
• fix: make Get / Set use short units like 'moment'
• fix: make 'ArraySupport' behave like moment
• fix: parse date returned "invalid Date" when parsing year only array
• fix: make plugin isYesterday use yesterday not today as reference
• feat: make add / subtract with decimal values behave like moment
• refactor: add type definitions for units for add / sub / diff / get / set
• docs: add comments to 'date' and 'day'
• refactor: make tests use 'test' instead of 'it' and add 'test.todo'
• test: fix mocking of 'new Date()'
• test: add tests for manipulating dayjs (taken from dayjs 1.x)
• test: extend constructor tests
• test: move ArraySupport tests in describe block of its own
• test: arrange tests for illegal values next to each other
• test: reset mockup function after test and remove redundant expects
• docs: add 'Changes' segment to README.md
• build: add setting "defaultFormatter" for vscode