Proc macro spans: make columns 1 based

[est31]

This makes proc macro spans consistent with the column!() macro as well as std::panic::Location, as both are 1-based.

#54725 (comment)

[rust-highfive]

r? @petrochenkov

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[est31]

Note that there are still small differences between proc macro span columns and panic location columns: the panic location counts a tab as 4 characters while the proc macro spans count it as 1 character. However, the column!() macro does this as well so I don't think it's a problem that needs solving.

[petrochenkov]

This looks reasonable and the API is (perpetually?) unstable, so I'm going to just r+ this.
@bors r+

[bors]

📌 Commit 7d20789 has been approved by petrochenkov

[bors]

⌛ Testing commit 7d20789 with merge 71ff9b4...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing 71ff9b4 to master...