[IMP] *: fix typos in field definitions

* account, crm, hr, im_livechat, l10n_ch, l10n_it_edi, mail,
mass_mailing, project, stock, base

- `String` -> `string`;
- `defaut` -> `default`;
- `defaults` -> `default`;
- `reandonly` -> `readonly`;
- removed redundant `placeholder` attribute for `Char` field;
- removed redundant `size` attribute for `Float` and `Integer` fields;
- changed to use `Selection` instead of `Char` for a field having a
defined `selection`.

closes odoo/odoo#35356

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>