feat: add function def/ref tags

quoting tree-sitter docs:
> Tree-sitter can be used in conjunction with its query language as a
> part of code navigation systems. [...] A notable application of this
> is GitHub's support for search-based code navigation.

assuming github is willing to continually run the indexing on a
repository with as much activity as nixpkgs, this should mean that
clicking a function on github allows you to navigate to/from the
definition of the function.

On my laptop, for nixpkgs:
> $ time bash -c "tree-sitter tags --paths <(git ls-files | rg '[.]nix$') >/dev/null 2>&1"
> bash -c   7.40s user 1.14s system 100% cpu 8.501 total

Signed-off-by: Nicholas Sielicki <[email protected]>

Author: sielicki

queries/tags.scm

@@ -0,0 +1,16 @@
+;; https://tree-sitter.github.io/tree-sitter/4-code-navigation.html
+
+;;Function definitions
+(binding
+  attrpath: (attrpath attr: (identifier)) @name
+  expression: (function_expression) @definition.function)
+;;Function/method calls
+(apply_expression function: (apply_expression function: (variable_expression name: (identifier) @name)) @reference.call)
+
+;; TODO: (if even applicable?)
+;;Interface definitions       @definition.interface
+;;Interface implementation    @reference.implementation
+;;Class definitions           @definition.class
+;;Class reference             @reference.class
+;;Method definitions          @definition.method
+;;Module definitions          @definition.module

tree-sitter.json

@@ -8,6 +8,7 @@
       "scope": "source.nix",
       "file-types": ["nix"],
       "injection-regex": "^nix$",
+      "tags": "queries/tags.scm",
       "class-name": "TreeSitterNix"
     }
   ],