Allow styling table cells using field formatters from core

We had a long discussion regarding the approach and agreed on the next things:

1. Keep the idea of having Formatter responsible for the styling. It looks much more flexible then pass the value in a specific format and then process it in Encoder.
2. Try to avoid this strpos and unserialize

   +++ b/src/Encoder/Xls.php
   @@ -253,13 +255,26 @@ class Xls implements EncoderInterface {
   +        if (strpos($value, ViewsExcelStyledFieldItemValue::class . '":') !== FALSE) {
   +          $styled = unserialize($value, [ViewsExcelStyledFieldItemValue::class]);
   

3. To achieve 2. one of the options is to write a RowPlugin that will skip serialization for fields with formatter ViewsExcelStyledFieldCellFormatterInterface so it can be processed in Ecoder.

Considering the proposed solution works and except for the hack with strpos and unserialize looks good to me marking as RTBC.
In the end, good enough is enough.

Adjusted #10 to be applied to the latest stable version.

Cleaning old issues from previous releases.
We can include it, when someone provides a MR or patch that can be applied to current codebase, we'll test and merge!

Implemented some basic Excel formatters (string, datetime, timestamp) that add new configuration options allowing to collapse/hide columns, define horizontal alignment, set font bold or italic.

Any feedback will be greatly appreciated.

Just added an ExcelImageFormatter allowing to insert images into your exported Excel files:

This can now be tested in the new Excel Serialization Extras sandbox module.