perf: dedupe calls to arena.create_node()

[bartveneman]

Roughly 4% uncompressed JS file savings.

before

> @projectwallace/css-parser@0.6.2 build
> vite build && tsc --project tsconfig.build.json

vite v7.2.2 building client environment for production...
✓ 15 modules transformed.
dist/tokenize.js               0.33 kB │ gzip: 0.23 kB
dist/index.js                  2.70 kB │ gzip: 0.95 kB
dist/parse-value.js            8.25 kB │ gzip: 1.80 kB
dist/parse-anplusb.js          8.85 kB │ gzip: 1.62 kB
dist/parse.js                 16.02 kB │ gzip: 3.20 kB
dist/lexer-CtBKgfVv.js        17.16 kB │ gzip: 2.94 kB
dist/parse-atrule-prelude.js  18.80 kB │ gzip: 3.10 kB
dist/css-node-aIMm9_Cb.js     23.50 kB │ gzip: 5.54 kB
dist/parse-selector.js        27.00 kB │ gzip: 4.71 kB
✓ built in 123ms

~/www/css-parser main
❯ du -sh dist
204K    dist

after

> @projectwallace/css-parser@0.6.2 build
> vite build && tsc --project tsconfig.build.json

vite v7.2.2 building client environment for production...
✓ 15 modules transformed.
dist/tokenize.js               0.33 kB │ gzip: 0.23 kB
dist/index.js                  2.70 kB │ gzip: 0.95 kB
dist/parse-value.js            8.25 kB │ gzip: 1.80 kB
dist/parse-anplusb.js          8.85 kB │ gzip: 1.62 kB
dist/parse-atrule-prelude.js  16.01 kB │ gzip: 2.87 kB
dist/parse.js                 16.02 kB │ gzip: 3.20 kB
dist/lexer-CtBKgfVv.js        17.16 kB │ gzip: 2.94 kB
dist/parse-selector.js        23.44 kB │ gzip: 4.53 kB
dist/css-node-aIMm9_Cb.js     23.50 kB │ gzip: 5.54 kB
✓ built in 120ms

~/www/css-parser dedupe-create-node-calls
❯ du -sh dist
196K    dist