feat(workbench): allow for a layout with an optional main area

The layout can be divided into a main and a peripheral area, with the main area as the primary place for opening views. The peripheral area arranges parts around the main area to provide navigation or context-sensitive assistance to support the user's workflow. Defining a main area is optional and recommended for applications requiring a dedicated and maximizable area for user interaction.

PR Checklist

Please check if your PR fulfills the following requirements:

[x] The commit message follows our guidelines
[x] Tests for the changes have been added
[x] Docs have been added or updated

PR Type

What kind of change does this PR introduce?

[ ] Fix
[x] Feature
[ ] Documentation
[ ] Refactoring (changes that neither fixes a bug nor adds a feature)
[ ] Performance (changes that improve performance)
[ ] Test (adding missing tests, refactoring tests; no production code change)
[ ] Chore (other changes like formatting, updating the license, removal of deprecations, etc)
[ ] Deps (changes related to updating dependencies)
[ ] CI (changes to our CI configuration files and scripts)
[ ] Revert (revert of a previous commit)
[ ] Release (publish a new release)
[ ] Other... Please describe:

Issue

Issue Number: #443

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Adding support for optional main area introduced a breaking change.

The following APIs have changed:

MAIN_AREA_PART_ID -> MAIN_AREA
WorkbenchModuleConfig.layout was changed to explicitly add an initial part e.g. MAIN_AREA

The APIs for the following directives have changed:

wbPartAction inputs view, part and area were replaced by canMatch function

SchweizerischeBundesbahnen / scion-workbench