vim: Fix substitute command for /n and /c

[lsparrish]

Closes #23345

Release Notes:

• Fixed vim substitute command to enable override of global (replace all) default behavior using flags /n and /c

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[lsparrish]

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⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

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[ConradIrwin]

Thanks for taking a pass at this!

I think we need to:

• Do no replacements with /n, it should be equivalent to just running a search
• For c I think we should probably do no replacements by default, but put the focus into the replacement bar so that <enter> can be used to replace things one by one, and make it so that hitting enter there doesn't move focus back to the buffer.
  Happy to pair on this if you want https://cal.com/conradirwin/pairing

[ConradIrwin]

Closing this for now, but still happy to pair on the implementation if you'd like