strings - Expand concept by newline and text blocks

[fapdash]

pull request

• Add multi-line strings / text blocks (Java 15)
• Add explanation of newlines / end of line chars, so students can understand the motivation behind text blocks

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Noticed that the String concept was lacking text blocks. While there are quite some details and intricacies to text blocks, they're not relevant for the level of detail the syllabus is aiming for, therefore it doesn't make sense to have them as an extra concept imo. I added infos about newlines so students can appreciate the motivation behind text blocks.

Looking forward to your feedback. :)

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