Feature request: make `Length` writable

[alex-jitbit]

Setting Length works great to trim a StringBuilder (like, "remove last N chars" without allocation)

Also, would really help with drop-in replacements for traditional SB (it supports this)

[alex-jitbit]

I've edited the issue, because seems like you're already using 65535 as the default size which should be enough (?)

[alex-jitbit]

I just realized that writable property won't work since it's a disposable struct, so maybe add a SetLength method

[github-actions[bot]]

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