Fix whitespace control

[jamslinger]

Checklist

• [x] I have read the contribution guidelines.
• [x] make test passes.
• [x] make lint passes.
• [x] New and changed code is covered by tests.
• [ ] Performance improvements include benchmarks.
• [x] Changes match the documented (not just the implemented) behavior of Shopify.

Instead of keeping trimming flags on all tokens, introduce explicit trim tokens: TrimLeftTokenType and TrimRightTokenType, trimWriter configuration is then triggere via (n *TrimNode) render.

[jamslinger]

fixes: https://github.com/osteele/liquid/issues/79