Remove underscore from base64DecodeTable

[primoly]

Fixes #36

[tschneidereit]

Thank you for this, @primoly! Can you say why this is the right fix? I think it is, based on the table Firefox uses, but would feel better if you gave a rationale for this change that we can then check.

[primoly]

It’s specified in the table in Base 64 Encoding (Table 1: The Base 64 Alphabet) which does not contain the underscore character (ASCII 95).

[guybedford]

Can you add the test for this at the end of https://github.com/bytecodealliance/StarlingMonkey/blob/main/tests/integration/btoa/btoa.js demonstrating:

throws(() => atob("__"));