Add cmake.environmentSetupScript for preset mode

[vasdal]

This changes visible behavior + adds a setting

The following changes are proposed:

• Add a cross-platform cmake.environmentSetupScript setting. When using CMake
  Presets, CMake Tools sources (Linux/macOS) or calls (Windows) the script and
  uses its environment as the parent environment.
• The script env is applied to both $penv{}/include resolution and to
  per-preset expansion + the configure/build/test spawn, so a single script
  bootstraps everything. Cached by script path + mtime; re-applied on change.
• Reuses the kit's script-sourcing code (no behavior change for kits).

The purpose of this change

Presets can only auto-bootstrap a parent environment on Windows (vcvarsall via
tryApplyVsDevEnv). On other platforms there's no way to source a setup script,
so environments whose variables are large, interdependent, or produced by a
tool's own setup script can't be used with presets: $penv{...} in
include/toolchain paths doesn't resolve (presets may fail to load) and
find_package() fails at configure time. Reconstructing such environments as
static cmake.environment entries isn't viable; sourcing the script is — which
is exactly what kits already do. This brings that to preset mode,
cross-platform.

Other Notes/Information

• Same __parentEnvironment seam as the VS Dev Env; no new driver plumbing.
• Setting value supports ${workspaceFolder}-style substitution.
• eslint clean; yarn.lock untouched; no dependency changes.

[vasdal]

@microsoft-github-policy-service agree