Backport of cli: Improved error for invalid -var "foo = bar" into v1.2

[teamterraform]

Backport

This PR is auto-generated from #30985 to be assessed for backporting due to the inclusion of the label 1.2-backport.

The below text is copied from the body of the original PR.

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When specifying variable values on the command line, name-value pairs must be joined with an equals sign, without surrounding spaces. Previously Terraform would interpret -var "foo = bar" as assigning the value " bar" to the variable named foo . This is never valid, as variable names may not include whitespace.

This commit looks for this specific error and returns a diagnostic with a suggestion for correcting it. We cannot simply trim whitespace, because it is valid to write -var "foo= bar" to assign the value " bar" to the variable foo, as unlikely as it seems.

Fixes #30600, see also documentation in #30969.

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