Add slice, urlEncode, base64Encode, base64Decode, and coalesce functions

[Copilot]

Adds five new expression functions as requested.

New Functions

• slice(s, start, end?) — Extract portion of string or array; supports negative indices
• urlEncode(str) — URL-encode via encodeURIComponent
• base64Encode(str) — Base64-encode with proper UTF-8 handling using btoa
• base64Decode(str) — Base64-decode with proper UTF-8 handling using atob
• coalesce(a, b, ...) — Return first non-null, non-empty-string value

Usage

const parser = new Parser();

// slice - works with strings and arrays, supports negative indices
parser.evaluate('slice("hello world", 0, 5)');     // "hello"
parser.evaluate('slice("hello world", -5)');       // "world"
parser.evaluate('slice([1,2,3,4,5], -2)');         // [4, 5]

// urlEncode
parser.evaluate('urlEncode("foo=bar&baz")');       // "foo%3Dbar%26baz"

// base64Encode / base64Decode - handles UTF-8
parser.evaluate('base64Encode("hello")');          // "aGVsbG8="
parser.evaluate('base64Decode("aGVsbG8=")');       // "hello"
parser.evaluate('base64Encode("日本語")');          // "5pel5pys6Kqe"
parser.evaluate('base64Decode("5pel5pys6Kqe")');   // "日本語"

// coalesce - first non-null/non-empty value
parser.evaluate('coalesce("", null, "found")');    // "found"
parser.evaluate('coalesce(null, 0, 42)');          // 0 (0 is valid)

All functions follow existing patterns: return undefined for undefined inputs, throw descriptive errors for type mismatches.

Original prompt

Add support for the following functions:

• slice(s, start, end?) (with support for negative indices) it should support strings and arrays
• urlEncode
• base64Encode
• coalesce(a, b, ...) (first non-null/non-empty string)

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[Sander-Toonen]

Add documentation for the new functions, also make sure the language server knows about the new functions

[Sander-Toonen]

Remove comment

[Copilot]

Removed in b0fb3f9.

[Copilot]

Add documentation for the new functions, also make sure the language server knows about the new functions

Added in b0fb3f9:

• Documentation for the new functions (slice, urlEncode, base64Encode, coalesce) in docs/syntax.md
• Language server documentation in src/language-service/language-service.documentation.ts with descriptions and parameter info for all four functions

[Sander-Toonen]

@copilot Simplify this function by using "btoa" instead. Also create and document a base64Decode function.

[Copilot]

Simplified base64Encode to use btoa and added base64Decode function using atob in c537880. Also added documentation and language server support for both functions, plus 7 new tests for base64Decode.