fix: disable HTML escaping of prompt compile inputs

[hassiebp]

Disable HTML escaping of prompt compile inputs

Changes

• Removes chevron as template-replacement library as it does not support disabling HTML escaping
• Introduced custom compile_template_string function with test suite
• Jinja2 has the following restrictions on variable names. If the prompt feature users decide to keep a variable unused and see it rendered within double curly braces, Jinja2 would throw an error as it cannot parse the variable name.