[FIX] web_editor: check method addNeutralStyle

Because of css limitation, the child of a parent that define a
text-decoration cannot "neutralize" that decoration defined by its
parent with a css property (ie. the text-decoration cannot be
overridden).

When a block define an inline style or any tag that have a class, the
method `formatSelection` tries to "neutralize" the style by calling
`addNeutralStyle`. As it is impossible to "neutralize" the style for
underline nor strikeThrough, there is no method `addNeutralStyle`
defined for those properties.

Before this commit the method `formatSelection` was crashing because
`addNeutralStyle` was not defined.

Task-3002123

closes odoo/odoo#105953

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>