Allow setting Document(base: ) to null, and fix the path to the generated client script when no <base> element exists.

jaspr_content 0.4.0

• The LiquidTemplateEngine is no longer const.
• Fixed resolution of partial files in LiquidTemplateEngine.
• Skip adding a <style> element for an empty ContentTheme.
• Fixed HtmlParser to treat html files as fragments.

jaspr_lints 0.5.0

• Extend prefer_html_methods lint to also suggest using text() and fragment() methods, and offer fixes for them.

• Changed Add styles assist to create a getter instead of variable.

• Update analyzer to ^8.0.0.

• Update custom_lint to ^0.8.0.