Allow to check and/or validate XML files on save

### Description

NetBeans allows to perform certain actions when a file is saved. For example, reformatting it or removing tailing white-spaces:


In the specific case of XML files, I think it would be really useful to automatically check that the file is well-formed. If we could also enable/disable the validation separately, it would be even better.

### Use case/motivation

Checking/validating XML files manually, after every edition, is repetitive and distracting. So much, that sometimes you forget to make that checks and some errors go unnoticed.

### Related issues

This feature request is inspired by the issue #4908, in which syntax errors are not being highlighted anymore when you move the cursor to a well-formed line.

If we had a feature like the one described here, I guess the impact of an issue like that would be very low, because the errors would be shown in the _Output_ panel after saving the file.

### Are you willing to submit a pull request?

No

### Code of Conduct

Yes

Start by reviewing NetBeans' existing actions that run on save and the XML editor behavior described in issue #4908. Determine how XML well-formedness checking and optional validation should be configured, then verify that save-time errors appear in the Output panel without blocking unrelated save actions.

Written by the indexing model from the issue text.