docs(spelling): standardize spelling for consistency

[OmarElzero]

Fixes #119

This PR standardizes the spelling of several words across the documentation and template files for better consistency.

• Defence has been changed to Defense.
• colours has been changed to colors.

This was identified by running cspell across the repository's markdown files.

[Copilot]

Pull request overview

This PR aims to standardize American English spelling across documentation/template fixtures to improve consistency, as identified by cspell.

Changes:

• Replaces “colours” → “colors” in template fixtures and updates corresponding snapshot output.
• Replaces “Defence” → “Defense” in an insurance template fixture and updates corresponding snapshot output.
• Introduces a structural change in optional-nested/template.md around the optional age rendering (beyond spelling).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
test/templates/good/playground/template.md	Spelling update for “colors” in the playground template fixture.
test/templates/good/optional-nested/template.md	Spelling update plus a behavioral change to how the age line is emitted (now wrapped in an #optional).
test/templates/good/insurance-sub-limits-liability/template.md	Spelling update for “Defense Costs” in the insurance sub-limits fixture.
test/snapshots/TemplateMarkInterpreter.test.ts.snap	Snapshot updates to match the template text/AST changes.

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[OmarElzero]

Just a quick note to clarify the history here. This branch had some tangled commits from a previous PR by mistake. I've cleaned it up now, and this PR only contains the spelling standardization as intended. The outdated review comment can be disregarded.