feat: add prerelease VSIX validation step

[madhur310]

Addresses PR review high priority item #4:

• Validates VSIXs are marked as prerelease when pre-release: true
• Checks extension.vsixmanifest for Microsoft.VisualStudio.Code.PreRelease flag
• Fails build if any VSIX is not properly marked
• Only runs validation when pre-release input is true
• Reuses VSIX file list from checksum step for efficiency

This ensures the --pre-release flag was properly passed to vsce package command.

[peternhale]

Approved. Solid, useful addition — verifies that --pre-release was actually honored by vsce before shipping. Core logic is correct: multiline output reuse of the VSIX file list is clean, the empty-guard is correct, and paths with spaces are handled.

Minor non-blocking notes for follow-up:

• PR description mentions extracting package.json / checking a -pre version suffix, but the implementation checks extension.vsixmanifest for the PreRelease property (which is actually the better approach). Worth updating the description.
• The grep -q short-circuit inside a command substitution under pipefail is slightly fragile, though it won't fire in practice given manifest sizes.
• Emoji log output is inconsistent with the plain-text style elsewhere in this workflow.