Don't fail on parse, analysis, runtime errors

[berekuk]

Squiggle programs don't have to fail when execution can partially succeed. (Normal programs have side effects, so continuing execution is dangerous, but Squiggle is pure).

Examples:

• when parse fails at line 100, we can still show all values from lines 1-99, and if the parser has recovered from an error, we can even continue execution below the failed line
• when types don't match, we can highlight failed variables, but execute everything else
• on runtime errors, we can store "VError" in the failed variable and proceed

One downside of this approach is that we'll consume more CPU: if your program is x = longComputation(); y = , and you're still typing the value for y, it'll run x more times.

But this can be improved with:

• partial caches based on hashes of AST nodes
• web workers
• disabling autoruns, since it's only a problem when your entire program is slow