[LiquidDoc] Parsing support for optional parameters and default values

----
- Added a parameter `required` to the LiquidDocParamNode
- By default, parameters are required unless they have `[]` around the name
- Parameters with incomplete delimiters `e.g. ([missingTail)` will map to a `TextNode`
- Default values are supported for optional parameters - `[param=default]`
- Default values will match anything between `=` and `]`. Leading and Trailing spaces are trimmed.

I originally tried accessing everything from the top level in the param node.
This is still the goal, but I think this should be done at stage 2 so that we can leverage the recursive nature of how to-AST works.
My original approach attempted to Write everything to the top level in stage 1, but we should be doing this in stage 2.

Author: jamesmengo

.changeset/chatty-scissors-relax.md

@@ -0,0 +1,6 @@
+---
+'@shopify/liquid-html-parser': minor
+'theme-check-vscode': minor
+---
+
+Add support for parsing optional parameters in LiquidDoc. Optional parameters are designated by wrapping the name in square brackets, and can accept a default value (e.g. `@param [parameter_name=default]`).

packages/liquid-html-parser/grammar/liquid-html.ohm

@@ -398,14 +398,28 @@ LiquidDoc <: Helpers {
   strictSpace = " " | "\t"
   // We use this as an escape hatch to stop matching TextNode and try again when one of these characters is encountered 
   openControl:= "@" | end
+  // We use this to map text values into a textNodes via mappings in stage-1-cst.ts
+  textValue = identifierCharacter+
 
   fallbackNode = "@" anyExceptStar<endOfParam>
-  paramNode = "@param" strictSpace* paramType? strictSpace* paramName (strictSpace* "-")? strictSpace* paramDescription
+  paramNode = "@param" strictSpace* paramType? strictSpace* (paramName | optionalParamName) (strictSpace* "-")? strictSpace* paramDescription
+
   paramType = "{" strictSpace* paramTypeContent strictSpace* "}"
   paramTypeContent = anyExceptStar<("}"| strictSpace)>
-  paramName = identifierCharacter+
-  paramDescription = anyExceptStar<endOfParam>
+
+  paramDescription = (~"]" anyExceptStar<endOfParam>)
   endOfParam = strictSpace* (newline | end)
+
+  // Required parameter name - e.g. "paramName"
+  paramName = textValue
+  // Optional parameter name - e.g. "[paramName]"
+  optionalParamName = "[" strictSpace* optionalParamNameContent strictSpace* "]"
+  // Content of an optional parameter name - e.g. "paramName=defaultValue"
+  optionalParamNameContent = strictSpace* textValue strictSpace* optionalParamNameDefaultValue?
+  // Default value of an optional parameter name - e.g. "=defaultValue"
+  optionalParamNameDefaultValue = "=" strictSpace* optionalParamNameDefaultContent
+  optionalParamNameDefaultContent = anyExceptPlus<endOfOptionalParamNameDefaultValue>
+  endOfOptionalParamNameDefaultValue = strictSpace* "]"
 }
 
 LiquidHTML <: Liquid {