lang/funcs: add acc tests for functions (#21112)

* lang/funcs: testing of functions through the lang package API
The function-specific unit tests do not cover the HCL conversion that happens when the functions are called in a terraform configuration. For e.g., HCL converts sets to lists before passing it to the function. This means that we could not test passing a set in the function _unit_ tests.
This adds a higher-level acceptance test, plus a check that every (pure) function has a test.

* website/docs: update function documentation

.tfdev

@@ -0,0 +1,12 @@
+version_info {
+  commit_var  =    "main.GitCommit"
+  version_var =    "github.com/hashicorp/terraform/version.Version"
+  prerelease_var = "github.com/hashicorp/terraform/version.Prerelease"
+}
+
+version_exec = false
+
+platform {
+    os = "darwin"
+    arch = "amd64"
+}