Add JSON Linter to Online tools

[enudler]

Adds jsonlinter.dev to the Online tools section.

What it is

A single-page JSON validator and formatter that runs entirely in the
browser. The whole site is a ~9KB HTML file — no framework runtime,
no analytics, no tracking.

Distinguishing features vs. tools already on the list:

• Line and column error positions — V8/Safari report errors as a
  character offset; the linter walks the input to translate to
  line:column so the error is easy to find.
• Format / minify with configurable indent (2, 4, or tab).
• Works offline once loaded.
• No ads or signup walls.

Placed next to JSON.fr since the pitch is similar
(client-side validator/formatter); happy to move if a different spot
fits better.

Contributing-guide compliance

• Single line addition, alphabetic-ish placement near similar tools
• Format: * [Name](url) - description ending with a dot.
• No mention of "JSON" in the description (implied)
• No trailing whitespace
• Link works (HTTP 200, valid TLS)

Live link

https://jsonlinter.dev/