PR: Calculate index when autoformating on save from the Editor

[dalthviz]

Description of Changes

• Wrote at least one-line docstrings (for any new functions)
• Added unit test(s) covering the changes (if testable)

• Included a screenshot or animation (if affecting the UI, see Licecap)

Issue(s) Resolved

Fixes #14639

Affirmation

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with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

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[ccordoba12]

Thanks for your work on this @dalthviz! The bug reported in #14639 is fixed, but there's a small glitch: after the file is saved, it's still shown as dirty in the interface (i.e. marked with *):

[ccordoba12]

Looks good to me now, thanks @dalthviz!