feat: static variable analysis (#770)

* feat: static variable analysis

* Accept any iterable from `children`, `arguments`, etc.

* Test analysis of standard tags

* Use `TagToken.tokenizer` instead of creating a new one

* Test analysis of netsted tags

* Group variables by their root value

* Test analysis of nested globals and locals

* Analyze included and rendered templates WIP

* Use existing tokenizer when constructing `Hash`

* Improve test coverage

* Analyze variables from `layout` and `block` tags

* Test analysis of Jekyll style includes

* Handle variables that start with a nested variable

* Async analysis

* Test non-standard tag end to end

* Implement convenience analysis methods on the `Liquid` class

* More analysis convenience methods

* Accept string or template array

* Draft static analysis docs

* Deduplicate variables names

* Fix isolated scope global variable map

* Coerce variables to strings instead of extending String

* Private map instead of extending Map

* Fix e2e test

* Tentatively implement analysis of aliased variables

* Fix nested variable segments array

* Update docs sidebar

src/util/underscore.ts

@@ -194,3 +194,16 @@ export function argumentsToValue<F extends (...args: any) => any, T> (fn: F) {
 export function escapeRegExp (text: string) {
   return text.replace(/[-[\]{}()*+?.,\\^$|#\s]/g, '\\$&')
 }
+
+/** Return an array containing unique elements from _array_. Works with nested arrays and objects. */
+export function * strictUniq<T> (array: Array<T>): Generator<T> {
+  const seen = new Set()
+
+  for (const element of array) {
+    const key = JSON.stringify(element)
+    if (!seen.has(key)) {
+      seen.add(key)
+      yield element
+    }
+  }
+}