feat: add support for custom tag types

[bgub]

Summary

• Adds a customTags config option for defining custom tag prefixes with handler functions
• Keys are tag prefixes, values are functions that receive the tag content (as a string) and the template data object
• Supersedes feat: Add suport for custom tag types #364 by @Tikolu — original commit preserved with cleanup on top

Usage

const translations = {
  en: { greeting: "Hello!" },
  pl: { greeting: "Cześć!" },
};

const eta = new Eta({
  customTags: {
    "#": () => "",  // comment tag
    "*": (key, data) => translations[data.lang][key.trim()],
  },
});

eta.renderString("<%# comment %><p><%* greeting %></p>", { lang: "en" });
// => "<p>Hello!</p>"

Changes

• src/config.ts — add customTags to EtaConfig with default {}
• src/parse.ts — include custom tag prefixes in the parser regex, set tag type to the prefix string
• src/compile-string.ts — dispatch custom tag types in compileBody via this.config.customTags
• test/render.spec.ts — 4 tests (basic lookup, comment, multiple tags, mixed with built-in tags)

Test plan

• Full suite (106 tests) passing
• pnpm format, pnpm lint, pnpm build all clean

Closes #364

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.35%. Comparing base (acff516) to head (299f09e).

Files with missing lines	Patch %	Lines
src/compile-string.ts	66.66%	0 Missing and 1 partial ⚠️
src/parse.ts	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #368      +/-   ##
==========================================
+ Coverage   92.15%   92.35%   +0.20%     
==========================================
  Files          11       11              
  Lines         293      301       +8     
  Branches       78       81       +3     
==========================================
+ Hits          270      278       +8     
  Misses          9        9              
  Partials       14       14              

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