fix(treesitter): deduplicate repeated breadcrumb symbols

[disrupted]

When using the Treesitter source with nested mapping-style structures, dropbar often shows duplicated breadcrumb entries.

For example when using this Nickel configuration file

{
  languages = {
    nu = {
      extensions = ["nu", "nuon"],
      grammar.source.git = {
        git = "https://github.com/nushell/tree-sitter-nu.git",
        rev = "bb3f533e5792260291945e1f329e1f0a779def6e",
      },
    },
  },
}

the breadcrumbs can look like this:

languages.ncl> languages> languages> nu> nu> grammar.source.git> grammar.source.git> git> git

I've implemented heuristics that resolve a lot of those issues which previously lead to incorrect or noisy breadcrumbs. Now it should be more concise and preserve real nesting.

languages.ncl> languages> nu> grammar.source.git> git

[disrupted]

I wasn't sure if real Treesitter parsers can be used in CI. using stubs for treesitter instead makes it more portable.