feat: add "experimental" key to parsers

feat(ci): mark parsers as experimental in README

tests/unit/parsers_spec.lua

@@ -0,0 +1,46 @@
+local stub = require "luassert.stub"
+local parsers = require "nvim-treesitter.parsers"
+
+describe("maintained_parsers", function()
+  before_each(function()
+    stub(vim.fn, "executable")
+  end)
+
+  after_each(function()
+    vim.fn.executable:clear()
+  end)
+
+  it("does not return experimental parsers", function()
+    local old_list = parsers.list
+    parsers.list = {
+      c = {
+        install_info = {
+          url = "https://github.com/tree-sitter/tree-sitter-c",
+          files = { "src/parser.c" },
+        },
+        maintainers = { "@vigoux" },
+      },
+      d = {
+        install_info = {
+          url = "https://github.com/CyberShadow/tree-sitter-d",
+          files = { "src/parser.c", "src/scanner.cc" },
+          requires_generate_from_grammar = true,
+        },
+        maintainers = { "@nawordar" },
+        experimental = true,
+      },
+      haskell = {
+        install_info = {
+          url = "https://github.com/tree-sitter/tree-sitter-haskell",
+          files = { "src/parser.c", "src/scanner.cc" },
+        },
+      },
+    }
+
+    local expected = { "c" }
+
+    assert.same(parsers.maintained_parsers(), expected)
+
+    parsers.list = old_list
+  end)
+end)