Fix final-word lookup without source offsets - #2178

When a custom syntax provides only line/column positions, a word ending at the node's last character is not found. positionBy() falls back to the node start, and rangeBy() makes errors and warnings point there too.

For example, with postcss-sass@0.5.0:

let sass = require('postcss-sass')
let decl = sass.parse('a\n  color: red').first.first

decl.positionBy({ word: 'red' }).column // 3, expected 10
decl.rangeBy({ word: 'red' }).start.column // 3, expected 10
decl.error('Bad color', { word: 'red' }).column // 3, expected 10

The end line/column is inclusive, but slice() needs an exclusive end. This reuses the end offset already normalized by rangeBy() for both word searches, without adding another conversion rule.

The existing missing-offset tests now also check a word at the node end. Both fail before the change and pass afterward. pnpm test passes all 721 tests, coverage, lint, types, version, integration, and size checks.

The measured columns for the example above: