Implement evalTpl, a dependency-free TypeScript evaluator for a subset of
Go's html/template, used for browser preview of exported templates.
Support: {{.Field}} / {{.A.B}}, $ and $.Field, {{if}}…{{else}}…{{end}}, {{range}}…{{else}}…{{end}} (dot rebinds to element), pipelines of and/or/not/eq/ne, string/number/bool literals, parenthesized expressions. Use Go truthiness (zero value falsy; string "0" is truthy). HTML-escape interpolated values; render missing values as ''. Never throw: anything outside the subset passes through verbatim.