feat: support escaped expressions

[mxstrand]

Proposed changes

Hatter uses {{ }} for expression evaluation (e.g., {{ faker.lastName() }}). However, some use cases require outputting literal {{ }} in fixture data - for example when fixtures contain template strings that use the same syntax for variable substitution at runtime.

Currently, there is no way to output literal {{ }} in a Hatter fixture. Any {{content}} is interpreted as an expression and evaluated.

This PR adds support for backslash escaping: \{{name}} outputs literal {{name}} without evaluation.

Usage Example

  tables:
    email_templates:
      rows:
        welcome:
          subject: "Welcome \{{username}}"
          body: "Hello \{{name}}, your account is ready."

Backward Compatibility

The backslash escape is new syntax that was previously invalid (would have caused an expression evaluation error), so no existing fixtures are affected.

Checklist

• I have read the Contributing doc
• I have read the Creating and reviewing pull requests at LinkORB guide doc
• Lint and unit tests pass locally with my changes
• I have added/updated necessary documentation in the README.md or doc/ directories (if appropriate)

Further comments