Extend Manage Solution dialog with compiler selection

[jkrech]

Moved here from ARM-software/vscode-cmsis-csolution#461

Summary

If a select-compiler node is present in a csolution file, the Manage Solution dialog in the csolution extension should present the listed compilers as selectable options and allow the user to choose the active compiler.

Problem

Currently, even when a solution explicitly defines multiple supported compilers, users must switch compilers outside the Manage Solution workflow (e.g. by editing files or using external configuration). This makes compiler selection less discoverable and increases configuration friction. Users that transition from one compiler to another (e.g. AC6 to ATfE) are a typcial use-case for users switching compilers more frequently in a single solution.

Proposed Solution

When the csolution file contains a select-compiler node, the extension should:
• Detect the available compilers (select-compiler tag)
• Populate the Manage Solution dialog with these compilers
• Allow the user to select one as the active compiler

Example csolution fragment:

select-compiler:
  - compiler: GCC
  - compiler: CLANG@21.1.1
  - compiler: AC6

Expected Behavior

Only compilers listed under select-compiler are shown

The selected compiler is applied to the solution configuration