Fix logical error in text.tact #603

[Srg-12]

The fromBase64 function contains a logical error in the if conditions used to check for specific characters. Instead of using the logical AND operator &&, the code uses the "||" to combine conditions. This will result in a syntax error.

Original:
} else if (code == 45 || code == 43) { // - or +
result = result.storeUint(62, 6);
} else if (code == 95 || code == 47) { // _ or /

Improved:
} else if (code == 45 && code == 43) { // - or +
result = result.storeUint(62, 6);
} else if (code == 95 && code == 47) { // _ or /

Closes tact-lang#603

• I have updated CHANGELOG.md
• I have documented my contribution in Tact Docs: https://github.com/tact-lang/tact-docs/pull/PR-NUMBER
• I have added tests to demonstrate the contribution is correctly implemented: this usually includes both positive and negative tests, showing the happy path(s) and featuring intentionally broken cases
• I have run all the tests locally and no test failure was reported
• I have run the linter, formatter and spellchecker
• I did not do unrelated and/or undiscussed refactorings

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