Tracking Issue: Support Fleet Configuration in ./x.py setup

[HTGAzureX1212]

This is currently blocked by Fleet itself. Waiting on Fleet to release their 1.30.whatever version, which supposedly allows user to override the version of rust-analyzer used by Fleet to fix the procedural macro server invocations.

Hence, the milestone for this will be updated as time goes, until that specific version is released.

Steps:

• Initial Implementation x.py setup: Add Fleet Configuration #1988
• Add rust-analyzer.linkedProjects

[HTGAzureX1212]

Specifying linked projects is currently blocked by weird behaviour of Fleet starting the LSP in its installation directory, temporarily neglecting that in the current implementation: https://youtrack.jetbrains.com/issue/FL-24692/Specifying-rust-analyzer.linkedProjects-Break-due-to-Path