feat(lsp): st-import specifier completions

[idoros]

This PR adds path completions to the @st-import specifier.

• [x] relative paths
• [x] custom module resolver (returns both the resolved and original specifier to allow nested paths to be suggested)
• [x] package names (including scoped packages)
• [x] relative paths into packages with no exports field
• [x] naive exports field completions
  • complete wild card with no filtering after
  • default hard coded accepted conditional fields with simple dfs rule finding ("import", "require", "node", "browser", "default")
  • no null value filtering from result suggestions