Multiple fields support

I could not find it clearly written on the specs, but HTML allows an input file to have several values. For instance this is valid HTML:

<input type="text" name="foo" value="bar">
<input type="text" name="foo" value="baz">

However, wtforms does not support this, and always consider the first item of the request for a given name (here bar).
https://github.com/pallets-eco/wtforms/blob/9f9146c8ee2cb2b2e7b3d9bc09ac7e433fd55bb0/src/wtforms/fields/core.py#L362-L363

wtforms provides an alternative to manage several times the same input, with FieldList. However FieldList have some differences:

• the input have an incremental suffix, so this makes JS reordering of the fields more complicated for instance
• resizing of the data list must be done with append_entry and pop_entry, users cannot directly delete a field in the middle of the list
• it allows to nest FormField and make complex forms.

I think wtforms could support multiple fields. For instance we could add a multiple field in Fields with a default value at False, matching the current behavior. If set to True field.data would return a list instead of a single value. Validators should validate each individual values.

This could help with the validators handle one single value part of #662

Start in src/wtforms/fields/core.py at the request-value handling around lines 362-363, and compare the existing FieldList behavior and the concerns linked from issue #662. Determine the supported API for a multiple field, including how field.data and validators should behave, then add coverage showing repeated names are handled without changing the default behavior.