Introduce support to range iterations and SFR tags

[karreiro]

Fixes https://github.com/Shopify/theme-check/issues/681, https://github.com/Shopify/theme-check/issues/682, and https://github.com/Shopify/theme-check/issues/684 (I've opened a single PR because the fixes intersect a bit in the code, and I'd have dependent PRs)

---

• https://github.com/Shopify/theme-check/issues/681

  • This PR fixes this issue by introducing support to SFR tags in the TolerantParser

• https://github.com/Shopify/theme-check/issues/682

  • The AssignmentsCompletionProvider no longer crashes when it finds an unexpected lookup

• https://github.com/Shopify/theme-check/issues/684

  • The Scope class now supports literal ranges

[karreiro]

Thank you, @Poitrin! Excellent catch 🚀

I've applied the fix in a new commit :)