[Proposal] Change default _PROMPT from "> " to "> " (two spaces)

["Soni \"They/Them\" L." <fakedme@...>]

Hi! As you may or may not know, Lua uses _PROMPT and _PROMPT2 in the 
standard REPL (indeed, it's described in the reference manual).

Their default values, e.g. when unset, are "> " and ">> ". There are 
some reasons why this isn't good:

1. Terminals have block selection mode (hold control while selecting). 
The different widths makes this difficult.
2. That's about the only reason.

I propose we change this so the default/fallback _PROMPT and _PROMPT2 
are ">  " and ">> " respectively, i.e. they have the same width by 
default. This would work really well with block selection.

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