Add comprehensive changeset documentation for performance optimizations (#84)

* Initial plan

* Update changeset with comprehensive PR analysis and detailed optimization descriptions

Co-authored-by: hckhanh <[email protected]>

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: hckhanh <[email protected]>

Author: Copilot

.changeset/petite-pumas-flash.md

@@ -2,4 +2,15 @@
 "fast-url": patch
 ---
 
-[Copilot, update this]
+Performance optimizations for path and parameter handling utilities in `src/index.ts`. This release focuses on reducing regex recompilation overhead, optimizing string manipulation for path joining, and improving parameter filtering performance.
+
+**Performance Optimizations:**
+
+- **Pre-compiled regex**: Extracted the path parameter regex (`PATH_PARAM_REGEX`) to module scope to avoid recompiling it on every `path()` call, improving efficiency for path template processing.
+- **Optimized `join()` function**: Rewrote to use direct string indexing (`part1[len1 - 1]`, `part2[0]`) instead of `endsWith`/`startsWith` methods, added fast paths for empty strings, and optimized for the most common URL joining scenario where both parts have separators. This reduces unnecessary string slicing and improves speed.
+- **Optimized `removeNullOrUndef()` function**: Improved performance by checking for null/undefined values before allocating a new object, and using direct property iteration instead of `Object.entries`/`Object.fromEntries`. This results in faster execution and less memory usage, especially when no filtering is needed.
+
+**Parameter Validation Improvements:**
+
+- Enhanced `validatePathParam()` to check for empty strings in path parameters, ensuring that string path parameters cannot be empty or whitespace-only values.
+- Improved code readability by adding blank lines between logical blocks in validation logic.