feat(element-template): delete outdated templates

[peterhnm]

• Add the clean option to the plugin
• Append test class with a test that creates files in the output directory before the generation and checks if these files are deleted after the execution of the plugin
• Update README.md

Closes #391

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Project coverage is 46.03%. Comparing base (6bb0c6f) to head (1a445c2).

Files	Patch %	Lines
.../elementtemplate/ElementTemplateGeneratorMojo.java	71.42%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #391 +/- ## ============================================ + Coverage 45.77% 46.03% +0.26% - Complexity 393 396 +3 ============================================ Files 187 187 Lines 2073 2083 +10 Branches 110 110 ============================================ + Hits 949 959 +10 - Misses 1088 1090 +2 + Partials 36 34 -2 ``` | [Flag](https://app.codecov.io/gh/Miragon/miranum/pull/391/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Miragon) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/Miragon/miranum/pull/391/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Miragon) | `46.03% <71.42%> (+0.26%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Miragon#carryforward-flags-in-the-pull-request-comment) to find out more.

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