[FIX] web_editor: have transcoder apply text-decoration

The transcoder (transforming CSS rules into inline CSS for mail clients)
was not working for text-decoration property because it would transform
text-decoration values in:

{'text-decoration-thickness': 'normal', 'text-decoration': none}

And the regex that ensures we don't add duplicates check if there is:

"(^|;)\s*text-decoration"

so if there is text-decoration-thickness, text-decoration is not added.

The regexp could be changed to:

"(^|;)\s*text-decoration[^\w-]"

but we already have special case for other text-decoration-* properties.

With this change we should get text-decoration applied in:

- edit mode (was already the case)
- readonly mode (new)
- mail client (only if mail client does not override it)

As a side note: text-decoration-thickness seems to be rather new[^1] so
at one point in time there was probably no issue.

[^1]: https://developer.mozilla.org/docs/Web/CSS/text-decoration-thickness



opw-2496490

closes odoo/odoo#69074

X-original-commit: f4f1b890
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>
Co-authored-by: Kamen Zhekov <kzh@odoo.com>