[Fix] Prevent multiline values in short TextInputs

[BattleRush]

Description

When creating a Modal TextInput with the style Short and setting the value to a multi-line string, the error received will say just "Invalid Form Body", without exact feedback to the developer what the issue is.

// var description = <multi-line string>

var builder = new ModalBuilder()
{
    Title = "Custom modal",
    CustomId = "custom-id"
};

builder.AddTextInput("Description", "description", placeholder: "Description", value: description, required: true);

var modal = builder.Build();

// send modal

This will cause an error:

The server responded with error 50035: Invalid Form Body
COMPONENT_VALIDATION_FAILED: Component validation failed

Changes

In TextInputBuilder we check if the value set is a multi-line string by checking the presence of \n and if the Style is short we throw a ArgumentException.

Currently it would still be possible to cause this error by doing the following step:

1. Set Style to Paragraph
2. Set multi-line string
3. Change Style to Short

Additional checks could be set if requested to the setter of TextInputStyle property

[Misha-133]

This validation shouldn't be placed in the property setter, as it might be used before setting the style of the text input.
It should be added to the Build() method instead, this way we can be sure the style is set to the right value.

[BattleRush]

Moved to Build method