Add go-to-definition for variable references → assign tags

[jamesmengo]

What

When a user Cmd+Clicks (or an agent calls textDocument/definition) on a variable like {{ my_var }}, the LSP now jumps to the {% assign my_var = ... %} that defines it.

Why

This is the highest-value goToDefinition gap — no existing LSP feature provides this navigation today. It helps both human developers and AI agents navigate Liquid codebases.

How

New VariableDefinitionProvider registered alongside the existing translation definition providers:

1. Detects VariableLookup nodes with a non-null name
2. Visits the AST collecting all AssignMarkup nodes with matching name
3. Picks the last assign before the cursor offset (handles shadowing)
4. Returns a LocationLink from the variable reference to the assign tag

Shadowing example

{% assign x = 1 %}
{{ x }}              ← jumps to first assign
{% assign x = 2 %}
{{ x }}              ← jumps to second assign

Tests

9 new tests covering:

• Basic assign → reference navigation
• Variable shadowing (each reference resolves to correct assign)
• Global/contextual variables return null (no false positives)
• VariableLookup with name = null returns null
• Variable used before any assign returns null
• Variables with filters
• Multiple distinct variable names

All 13 definition tests pass (4 existing + 9 new).

Future work

This covers {% assign %} only. Follow-up tasks planned for:

• {% capture %}
• {% for %} / {% tablerow %} (block-scoped)
• @param (snippet parameters)
• {% increment %} / {% decrement %}