feat: add mekong chain

[vince0656]

PR overview

This PR introduces support for the Mekong devnet by adding a new chain definition and exporting it from the main chains index.

Detailed summary

• Added Mekong to the project.
• Created a new file src/chains/definitions/mekong.ts with the mekong chain definition.
• Defined properties such as id, name, nativeCurrency, rpcUrls, and blockExplorers.
• Exported mekong from src/chains/index.ts.

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PR-Codex overview

This PR introduces the Mekong chain to the project, enhancing the available blockchain options.

Detailed summary

• Added Mekong chain definition in src/chains/definitions/mekong.ts.
• Exported mekong from src/chains/index.ts.
• Defined Mekong with ID 7078815900, name Mekong Pectra Devnet, and specified its native currency and RPC URLs.
• Set Mekong as a testnet with a dedicated block explorer URL.

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🦋 Changeset detected

Latest commit: 8ce7892

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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