Add treesitter textobjects (#728)

* Add treesitter textobject queries

Only for Go, Python and Rust for now.

* Add tree-sitter textobjects

Only has functions and class objects as of now.

* Fix tests

* Add docs for tree-sitter textobjects

* Add guide for creating new textobject queries

* Add parameter textobject

Only parameter.inside is implemented now, parameter.around
will probably require custom predicates akin to nvim' `make-range`
since we want to select a trailing comma too (a comma will be
an anonymous node and matching against them doesn't work similar
to named nodes)

* Simplify TextObject cell init

4 files changed, 46 insertions, 3 deletions

diff --git a/book/src/SUMMARY.md b/book/src/SUMMARY.md
index 3fa8e067..56f50e21 100644
--- a/book/src/SUMMARY.md
+++ b/book/src/SUMMARY.md
@@ -8,3 +8,5 @@
   - [Keymap](./keymap.md)
   - [Key Remapping](./remapping.md)
   - [Hooks](./hooks.md)
+- [Guides](./guides/README.md)
+  - [Adding Textobject Queries](./guides/textobject.md)
diff --git a/book/src/guides/README.md b/book/src/guides/README.md
new file mode 100644
index 00000000..96e62978
--- /dev/null
+++ b/book/src/guides/README.md
@@ -0,0 +1,4 @@
+# Guides
+
+This section contains guides for adding new language server configurations,
+tree-sitter grammers, textobject queries, etc.
diff --git a/book/src/guides/textobject.md b/book/src/guides/textobject.md
new file mode 100644
index 00000000..50b3b574
--- /dev/null
+++ b/book/src/guides/textobject.md
@@ -0,0 +1,30 @@
+# Adding Textobject Queries
+
+Textobjects that are language specific ([like functions, classes, etc][textobjects])
+require an accompanying tree-sitter grammar and a `textobjects.scm` query file
+to work properly. Tree-sitter allows us to query the source code syntax tree
+and capture specific parts of it. The queries are written in a lisp dialect.
+More information on how to write queries can be found in the [official tree-sitter
+documentation](tree-sitter-queries).
+
+Query files should be placed in `runtime/queries/{language}/textobjects.scm`
+when contributing. Note that to test the query files locally you should put
+them under your local runtime directory (`~/.config/helix/runtime` on Linux
+for example).
+
+The following [captures][tree-sitter-captures] are recognized:
+
+| Capture Name       |
+| ---                |
+| `function.inside`  |
+| `function.around`  |
+| `class.inside`     |
+| `class.around`     |
+| `parameter.inside` |
+
+[Example query files][textobject-examples] can be found in the helix GitHub repository.
+
+[textobjects]: ../usage.md#textobjects
+[tree-sitter-queries]: https://tree-sitter.github.io/tree-sitter/using-parsers#query-syntax
+[tree-sitter-captures]: https://tree-sitter.github.io/tree-sitter/using-parsers#capturing-nodes
+[textobject-examples]: https://github.com/search?q=repo%3Ahelix-editor%2Fhelix+filename%3Atextobjects.scm&type=Code&ref=advsearch&l=&l=
diff --git a/book/src/usage.md b/book/src/usage.md
index 2de8d01a..d31e03a1 100644
--- a/book/src/usage.md
+++ b/book/src/usage.md
@@ -51,9 +51,10 @@ Multiple characters are currently not supported, but planned.
 
 ## Textobjects
 
-Currently supported: `word`, `surround`.
+Currently supported: `word`, `surround`, `function`, `class`, `parameter`.
 
 ![textobject-demo](https://user-images.githubusercontent.com/23398472/124231131-81a4bb00-db2d-11eb-9d10-8e577ca7b177.gif)
+![textobject-treesitter-demo](https://user-images.githubusercontent.com/23398472/132537398-2a2e0a54-582b-44ab-a77f-eb818942203d.gif)
 
 - `ma` - Select around the object (`va` in vim, `<alt-a>` in kakoune)
 - `mi` - Select inside the object (`vi` in vim, `<alt-i>` in kakoune)
@@ -62,5 +63,11 @@ Currently supported: `word`, `surround`.
 | ---                    | ---                      |
 | `w`                    | Word                     |
 | `(`, `[`, `'`, etc     | Specified surround pairs |
-
-Textobjects based on treesitter, like `function`, `class`, etc are planned.
+| `f`                    | Function                 |
+| `c`                    | Class                    |
+| `p`                    | Parameter                |
+
+Note: `f`, `c`, etc need a tree-sitter grammar active for the current
+document and a special tree-sitter query file to work properly. [Only
+some grammars](https://github.com/search?q=repo%3Ahelix-editor%2Fhelix+filename%3Atextobjects.scm&type=Code&ref=advsearch&l=&l=)
+currently have the query file implemented. Contributions are welcome !