[expo-file-system][next] Add base64(), rename path to uri.

[aleqsio]

Why

Mostly changes to variable names to make them clearer (we now use uri instead of path to better match file:/// URIs).

Also adding base64() function as an alternative to text().

How

Test Plan

Test suite

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).

[expo-bot]

The Pull Request introduced fingerprint changes against the base commit: b3addc3

Fingerprint diff

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    "op": "changed",
    "source": {
      "type": "dir",
      "filePath": "../../packages/expo-file-system/android",
      "reasons": [
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      "hash": "707adbe2a27148d95d72c53e9ddff20a2904432c"
    }
  },
  {
    "op": "changed",
    "source": {
      "type": "dir",
      "filePath": "../../packages/expo-file-system/ios",
      "reasons": [
        "expoAutolinkingIos"
      ],
      "hash": "fc967fbfde289430f3da90a22e393455e3510ece"
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  }
]

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