Simple builtin additions to String and Array functions

[OAGr]

This does a few things:

1. "foo" + "bar" + normal(3,3)
2. List.flatten() and List.join()
3. Forbid non-integer array lookups

I wasn't sure about some of this, happy to accept comments/suggestions.

This closes #1614

Also, this closes the comment to #1327

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Codecov Report

Merging #1669 (99ed3c3) into develop (72a77df) will increase coverage by 0.09%.
The diff coverage is n/a.

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@@             Coverage Diff             @@
##           develop    #1669      +/-   ##
===========================================
+ Coverage    74.00%   74.09%   +0.09%     
===========================================
  Files           93       93              
  Lines         5112     5134      +22     
  Branches       861      865       +4     
===========================================
+ Hits          3783     3804      +21     
- Misses        1304     1305       +1     
  Partials        25       25              

see 5 files with indirect coverage changes

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[OAGr]

Also should close: #843

[NunoSempere]

Could we also have List.append? Shouldn't be that different from List.join.

[berekuk]

"foo" + "bar" + normal(3,3)

I'm unsure about this. JS allows this, but Python doesn't ("abc" + 3 -> TypeError: can only concatenate str (not "int") to str), and JS infamously has many problems related to implicit casting.

I don't see anything terrible about this specific feature, though, just worried that having too many implicit casting features like this eventually combine into wats. So it seems safer to implement this only for strings for now.

Same concern about List.join: maybe it should be for lists of strings only, and in other cases we should ask for explicit list -> map(toString) -> join(',').

Note: string interpolation support to Squiggle (`foobar${normal(3,3)}`) is possible but somewhat hard, because of how parsers work (only some parsers are capable of recursively re-entering the parser from inside the string token; Peggy might be able to handle this, but I'm not sure).

[OAGr]

Could we also have List.append? Shouldn't be that different from List.join.

I'll do this in a different PR, is easy enough.

[OAGr]

I changed it so that List.join only works on lists of strings.

I think I want to keep the + coercion, especially because I could see it being a while until we have much better methods, like proper string introspection.

[berekuk]

I wanted to recommend Number.isInteger in place of lodash here, as I often do, but I benchmarked both versions and the performance is the same, so it doesn't matter.