Infrastructure: ignore .cache folder from Qt Creator's clangd

[vadi2]

Brief overview of PR changes/additions

Ignore .cache folder from Qt Creator's clangd, which is large and doesn't need to be checked in

Motivation for adding to Mudlet

Keeping things tidy

Other info (issues closed, discussion etc)

[add-deployment-links]

Hey there! Thanks for helping Mudlet improve. 🌟

Test versions

You can directly test the changes here:

• linux: https://make.mudlet.org/snapshots/beec19/Mudlet-4.18.5-testing-pr7421-4a6928a6-linux-x64.AppImage.tar
• osx: (download pending, check back soon!)
• windows 64 bit: https://make.mudlet.org/snapshots/35c4d1/Mudlet-4.18.5-testing-pr7421-4a6928a6-windows-64.zip
• windows 32 bit: https://make.mudlet.org/snapshots/62d184/Mudlet-4.18.5-testing-pr7421-4a6928a6-windows-32.zip

No need to install anything - just unzip and run.
Let us know if it works well, and if it doesn't, please give details.

[SlySven]

I'm approving this visually (without trying it out practically) as it seems to make sense in principle. As such I cannot tell whether the stuff in the .vscode/settings.json file works but the .gitignore looks good to me.