Support string enums in LiquidDoc parameter types

[clauderic]

What are you adding in this PR?

LiquidDoc @param types can now be string enums:

{% doc %}
  @param {'heading' | 'small'} [variant] - Text style
{% enddoc %}

• Theme Check accepts these declarations. It checks literal arguments to render (including with/for aliases), content_for and block against the allowed values, and suggests each allowed value as a replacement. When adding a missing required argument to render or content_for, it inserts the first allowed value. Dynamic values such as variables are not checked. Block parameters backed by a schema setting keep the setting's type, as in Validate block call parameters #1306.
• The language server keeps the allowed values through assignments and default, shows them in hovers and completion docs, and inserts the first one in render and content_for parameter completions.
• The parser no longer ends a LiquidDoc type at a } inside a quoted string. If a quote is left open, the type still ends at the first }, as before.

Enums are modeled as unions of string literal types rather than as a dedicated enum kind. Unions such as {string | number} or {product | collection} and discriminated unions (#223) can therefore build on this without another change to the model. For now, only string literals parse as union members.

The four commits are meant to be reviewed in order: parser fix, declarations, argument checks, language server.

What's next? Any followup issues?

• General unions such as {string | number} and {product | collection}: the parser has to accept type names as union members, and the checks and type system need rules for mixed unions.
• Discriminated unions: Support discriminated unions #223.
• Enums on schema-backed block parameters: the setting's type wins, so an enum declared for a text or select setting is not enforced at call sites yet.

Tophatting

In a snippet snippets/text.liquid that declares the @param above:

1. Hover {{ variant }}: it shows 'heading' | 'small'.
2. Hover style after {% assign style = variant | default: 'heading' %}: it still shows 'heading' | 'small'.
3. In a template, {% render 'text', variant: 'body' %} is reported, with suggestions to replace 'body' with 'heading' or 'small'.

• I added screenshots of the changes (before and after the changes if applicable)

Before you deploy

• I included a minor bump changeset
• My feature is backward compatible
• I included a patch bump changeset

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