Custom field / item "functions" should allow builtin function completions

[mghdotdev]

Describe the bug

Unable to complete builtin functions when writing code within item component "functions".

To Reproduce

Attempting to complete where the | is in the snippet below. Completion window only gives you more "component" function options.

<mvt:item name="customfields" param="Read_Category_Code( |, 'code', l.settings:category:customfield_values:customfields:code )" />