Adding a check to make sure 'standalone' extruder don't have buffer for pin_tool_start defined

[jimmyjon711]

Major Changes in this PR

• Added a check to make sure buffer is not defined for standalone toolheads as this is not a valid config.

Notes to Code Reviewers

How the changes in this PR are tested

Tested with toolchanger and created a standalone toolhead with buffer as pin_tool_start

PR Checklist: (Checked-off items are either done or do not apply to this PR)

• I have performed a self-review of my code
• CHANGELOG.md is updated (if end-user facing)
• Documentation updated in AT-Documentation repository
• Sent notification to software-design/software-discussions channel (as appropriate) requesting review
• If this PR address a GitHub issue, the issue number is referenced in the PR description

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