feat: add interpolate for string template transclusion

[char0n]

Summary

Implements the transclusion mechanism for string templates (#86) as interpolate().

interpolate(template, resolver, options?) renders Arazzo string templates by replacing each embedded {expression} with a value produced by a user-supplied resolver callback:

import { interpolate } from '@swaggerexpert/arazzo-runtime-expression';

interpolate('Hello {$inputs.name}!', () => 'world'); // => 'Hello world!'

const values = { '$inputs.clientId': 'abc', '$inputs.grantType': 'authorization_code' };
interpolate('client_id={$inputs.clientId}&grant_type={$inputs.grantType}', (expr) => values[expr]);
// => 'client_id=abc&grant_type=authorization_code'

Design

• Reuses the existing expression-string grammar/CST (same foundation as extract), so expression boundaries — including JSON Pointers in the name part — are parsed correctly rather than regex-matched.
• Because this package is a context-agnostic parser, resolution logic is supplied by the caller via resolver, which receives the raw expression (e.g. $inputs.name). This composes cleanly with a downstream evaluator.
• Resolved values are stringified via a configurable stringify option: defaults to '' for undefined/null, strings as-is, JSON.stringify for objects, String() otherwise.
• Throws TypeError for non-string templates / non-function resolvers; returns the template unchanged when it can't be parsed (mirroring extract's leniency).

Changes

• src/interpolate.js — implementation
• src/index.js — public export
• types/index.d.ts — interpolate, InterpolateResolver, InterpolateOptions typings
• README.md — new Interpolation section, TOC entry, updated tagline
• test/interpolate.js — 19 tests

Verification

Full npm run build succeeds with dual ESM/CJS output; all 188 tests pass; both built formats confirmed to export a working interpolate.

Closes #86

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a new interpolate() public API to render Arazzo “expression-string” templates by walking the existing CST for expression-string and replacing each {expression} with a caller-resolved value (with configurable stringification).

Changes:

• Implemented src/interpolate.js using the existing expression-string grammar + CSTTranslator.
• Exported interpolate from src/index.js and added TypeScript typings in types/index.d.ts.
• Documented interpolation in README.md and added a dedicated test suite in test/interpolate.js.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/interpolate.js	Core interpolation implementation + default stringification behavior.
src/index.js	Public export of interpolate.
types/index.d.ts	Adds interpolate typings plus resolver/options types.
test/interpolate.js	New tests covering interpolation behavior and edge cases.
README.md	Adds “Interpolation” usage docs and updates package capability list/TOC.

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[char0n]

🎉 This PR is included in version 3.1.0 🎉

The release is available on GitHub release

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