fix: serialize const string values to valid JSON

[spokodev]

Problem

A const string containing a backslash, control character, or double quote is serialized into corrupted or invalid JSON, even when the input matches the schema:

const build = require('fast-json-stringify')

build({ type: 'object', properties: { v: { const: 'a\\b' } } })({ v: 'a\\b' })
// -> {"v":"a\b"}            JSON.parse -> "a\b" (the value becomes a backspace)

build({ type: 'object', properties: { v: { const: 'a\tb' } } })({ v: 'a\tb' })
// -> {"v":"a<RAW TAB>b"}    JSON.parse -> SyntaxError: Bad control character

build({ type: 'object', properties: { v: { const: 'q"q' } } })({ v: 'q"q' })
// -> {"v":"q"q"}            JSON.parse -> SyntaxError

This reproduces for const at the top level, in properties, in array items, with type set, and as a nullable. enum and default with the same values serialize correctly — only const is affected.

Cause

JSON.stringify(schema.const) produces correct JSON text, but it is then embedded into a single-quoted JS string literal in the generated function source, escaping only single quotes (SINGLE_TICK):

code += `json += '${JSON.stringify(schema.const).replace(SINGLE_TICK, "\\'")}'`

When the Function constructor evaluates that source, \\ collapses back to \, a \t becomes a raw tab, and an embedded " breaks the JSON string. (Introduced by #658, which handled only the single-quote case.)

Fix

JSON.stringify the JSON text a second time to produce a properly escaped JS string literal — the same approach already used for default values:

code += `json += ${JSON.stringify(JSON.stringify(schema.const))}`

SINGLE_TICK is now unused and removed. This also still covers the original single-quote case from #658.

Tests

Added a case to test/const.test.js over ['back\\slash', 'tab\there', 'quote"here', 'new\nline', "tick's"] asserting the output equals JSON.stringify({ foo: value }) and round-trips (red before, green after).

Full unit suite green (473/473). Note: I ran it via node --test directly, because c8's bundled yargs cannot load under Node 26 (require is not defined in ES module scope) — unrelated to this change. npm run test:typescript passes.