Ensure that the IME interface methods are all defined for both browser
and webview modes. Deprecate 'Element' methods and replace with
'TextStore' to clarify return type. Add deprecation flags where needed
and update documentation accordingly.
Fixes: #16144
Note: build.sh updated to make sure that test:help build action:target
doesn't also call test.sh by adding `:_all` meta-target.
Note: CI will need a new help-keyman-com.sh build step to deploy web
engine help.
Fixes: #12347
