Count lines from 1 in error and warning locations

[marcoroth]

This pull request updates the locations in error messages and warnings to count lines from 1, so they point at the line with the problem instead of the one above it.

This follows Prism and Herb, which both count lines from 1. Columns in the messages already count from 1, the same as the column Herb prints in its messages, so they stay as they are.

css = ".a { color: red; }\n.b, { color: blue; }\n"

LightningCSS.transform(css, filename: "main.css")

- LightningCSS::ParseError: Invalid empty selector at main.css:1:5
+ LightningCSS::ParseError: Invalid empty selector at main.css:2:5

Resolves #17