theme-language-server-common: support go-to-definition for Liquid file references

[RomainDW]

What

Add textDocument/definition support for Liquid file references by reusing document-link targets.

Supported references:

• {% render 'snippet' %} / {% include 'snippet' %} -> snippets/<name>.liquid
• {% section 'name' %} -> sections/<name>.liquid
• {{ 'file.ext' | asset_url }} -> assets/<file.ext>
• {% content_for 'block', type: 'name' %} -> blocks/<name>.liquid

Why

Today these references are available through documentLink but not through definition, so cmd/ctrl+click go-to-definition does not work in editors that use textDocument/definition.

How

• Added DocumentLinksDefinitionProvider that:
  • computes all document links for the current file
  • selects the active link under cursor
  • returns a LocationLink for that target
• Wired it into DefinitionProvider
• Passed findThemeRootURI into DefinitionProvider from startServer

Tests

Added DocumentLinksDefinitionProvider.spec.ts covering:

• render snippet
• section
• asset_url
• content_for block type
• non-reference cursor returns null

Executed locally:

CI=1 corepack yarn test \
  packages/theme-language-server-common/src/definitions/providers/DocumentLinksDefinitionProvider.spec.ts \
  packages/theme-language-server-common/src/definitions/providers/TranslationStringDefinitionProvider.spec.ts \
  packages/theme-language-server-common/src/definitions/providers/SchemaTranslationStringDefinitionProvider.spec.ts

Result: 3 files passed, 6 tests passed.

Closes #1139