Add do snippet, structural snippets, and use-injection dedup (#73)

[JesseHerrick]

Closes #73

• Add "do" to elixirFormSnippets: expands to do..end block with tab stop
  when snippet support is enabled; falls back to plain keyword with
  Preselect when snippet support is off.
• Add structural snippets with parameter slots:
  defmodule, def, defp, defmacro, defmacrop
• Update if snippet to include else clause.
• Add test and describe snippets for ExUnit.
• Add elixirKeywords list: do and end appear as plain keyword completions
  to prevent VS Code from falling back to word-based suggestions.
• Add form-snippet dedup in addCompletionsFromUsing: use-injected
  functions (e.g. ExUnit.Case.test/1) no longer duplicate with their
  form snippet counterpart.

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Note

Low Risk
Changes are limited to LSP completion/snippet generation and parser token acceptance for def names; no auth, persistence, or runtime behavior outside the editor.

Overview
Improves Elixir LSP completions with richer snippets, smarter insert text, and fewer duplicate items.

Structural / special-form snippets expand elixirFormSnippets (e.g. defmodule, def/defp, defstruct, if with else, standalone do…end). Plain do/end keyword items (with Preselect) avoid VS Code word-based replacements when snippets are off or for simple keyword insert.

Use/import dedup: Kernel and addCompletionsFromUsing skip functions whose names match a form snippet so you get one keyword-style completion instead of a macro call duplicate.

Call-style completions: noParenFuncs emits space-separated args for ExUnit-style macros; doBlockSnippets (macros only via kind) produce full blocks for test, describe, setup, etc. applySnippet now takes kind to choose those templates.

Parser: isValidFuncNameToken treats tokens like defstruct as function names after defmacro, so Kernel-style defs index correctly for completion tests.

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