4/11: feat(treesitter): add helper module with navigation utilities

[juleswritescode]

Summary

• Add goto_node_at_position, goto_previous_leaf, goto_closest_unfinished_parent_clause
• Add previous_sibling_completed, last_children_completed
• Add SINGLE_TOKEN_RULES constant
• Export functions from lib.rs

Part of stacked PRs for #629

This is PR 4/11 in the keyword completion feature stack.

[psteinroe]

Can this be generated?

[psteinroe]

like what is this list / how is it defined?

[juleswritescode]

some treesitter nodes only ever have one child, such as literal:

program [0..12] 'select true;'
  statement [0..11] 'select true'
    select [0..11] 'select true'
      keyword_select [0..6] 'select'
      select_expression [7..11] 'true'
        term [7..11] 'true'
          literal [7..11] 'true'
            keyword_true [7..11] 'true'
  ; [11..12] ';'

So we're basically saying, "these token rules are always completed".

Alternatively, we could mark all children with an @end tag; I might do that later:

literal: ($) =>
      field("end", prec(
        2,
        choice(
          $._integer,
          $._decimal_number,
          $._literal_string,
          $._bit_string,
          $._string_casting,
          $.keyword_true,
          $.keyword_false,
          $.keyword_null
        )
      )),