Support let bindings in Effect blocks

[jmackie]

This allows pure variable binding in Effect blocks. For example:

main = do {
    let five: Int = 5;
    return five
};

Of course the above is a bit redundant (and should probably be rewritten as part of the JS codegen 👀 ) but this is useful in less trivial cases.

It's particularly useful as we don't currently support patterns on the left-hand side of effect binds (<-, due to parsing restrictions).

So eventually this will be nice for code like:

main = do {
    env <- get_env();
    let { foo } = env;
    return foo
};

Also note that as part of this I had to add a fresh variable supply to the JavaSCript code generator, so that we can use generated variables without risking bad const assignments (i.e. Identifier 'x' has already been declared).

[jmackie]

I don't necessarily want to do this because we might want to...

let _ = Debug.spy(some_value);

[jmackie]

This might blowup if we have something like...

let _ = 1;
let _ = 2;
let _ = 3;

In which case we could end generating dodgy const assignments, i.e.

const _ = 1;
const _ = 2;  // Identifier '_' has already been declared
const _ = 3;

[codecov]

Codecov Report

Merging #95 (0407ccc) into main (edc8829) will increase coverage by 0.12%.
The diff coverage is 87.73%.

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
+ Coverage   73.90%   74.03%   +0.12%     
==========================================
  Files         111      111              
  Lines        8319     8406      +87     
==========================================
+ Hits         6148     6223      +75     
- Misses       2171     2183      +12     

Impacted Files	Coverage Δ
crates/ditto-ast/src/expression.rs	81.63% <ø> (ø)
crates/ditto-codegen-js/src/optimize/mod.rs	95.30% <ø> (ø)
crates/ditto-fmt/src/has_comments.rs	44.74% <0.00%> (-1.63%)	⬇️
crates/ditto-codegen-js/src/convert.rs	95.33% <89.47%> (-1.11%)	⬇️
...ditto-checker/src/module/value_declarations/mod.rs	91.44% <100.00%> (+0.19%)	⬆️
crates/ditto-checker/src/typechecker/mod.rs	82.47% <100.00%> (+0.39%)	⬆️
crates/ditto-checker/src/typechecker/pre_ast.rs	100.00% <100.00%> (ø)
...ates/ditto-checker/src/typechecker/substitution.rs	78.07% <100.00%> (+0.59%)	⬆️
...ates/ditto-checker/src/typechecker/tests/effect.rs	100.00% <100.00%> (ø)
crates/ditto-cst/src/parser/tests/expression.rs	100.00% <100.00%> (ø)
... and 3 more

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