feat(filters): add squish filter

[MildlyMeticulous]

squish is a standard Shopify/liquid string filter and LiquidJS doesn't have it, so {{ "Hello World!" | squish }} currently renders unchanged rather than Hello World!.

engine.parseAndRenderSync('{{ "  Hello   World!  " | squish }}')
// before: "  Hello   World!  "
// after:  "Hello World!"

Strips leading and trailing whitespace and collapses each internal run to a single space, matching Shopify/liquid 5.12.0. Added alongside strip_newlines in src/filters/string.ts, with docs and a sidebar entry.

Verified against all 17 squish cases in golden-liquid, which tests against Shopify/liquid as the reference implementation. All pass. 1588 tests pass here; the new tests fail on revert.