Enhance CMSIS pack root access by introducing a fallback mechanism and improving error messages

[arneschmid]

Fixes

• #

Changes

• Fix path resolution for default and custom cmsis pack root folder

Screenshots

Checklist

• 🤖 This change is covered by unit tests (if applicable).
• 🤹 Manual testing has been performed (if necessary).
• 🛡️ Security impacts have been considered (if relevant).
• 📖 Documentation updates are complete (if required).
• 🧠 Third-party dependencies and TPIP updated (if required).

[brondani]

LGTM

In the long run we should consider improving code reuse across our repositories. For example, getCmsisPackRoot is already implemented in vscode-cmsis-solution:
https://github.com/Open-CMSIS-Pack/vscode-cmsis-solution/blob/0a7190fdeb44df0d9e08ed695701ede7b4af7cc2/src/utils/path-utils.ts#L50