[convert] On web, consider using atob and btoa to implement Base64 codec

[rakudrama]

It has been reported that Base64 decoding using the browser atob function followed by a copy loop to convert the result string into a Uint8List is 4-5x faster that using Base64.decode.

The main part of the decoder loop looks reasonable (other than the clumsy handling of outIndex++ which is not where the time is spent)

It seems that using atob will be worthwhile.

We did use TextDecoder to accelerate the decoding of UTF8 bytes into strings. Using the browser methods for Base64 conversion probably has similar issues:

• There may be incompatibilities.
• Errors may be reported differently (we might not care)
• Acceleration using the browser methods is probably slower for small inputs, and testing for the cutoff point slows down all conversions.