Add section "rename symbol"

[bendtherules]

Part of #2.

Only example added for discussion.
DONT MERGE now.

[itsdouges]

hey well this definitely accomplishes what we wanted! what's your thoughts on the PR in its current shape? (other than there isn't content in the handbook yet)

i like that its a good time to teach why you'd want to use forEachChild. i dislike that it's a lot of code though.

[bendtherules]

Need to remove some console.logs.

Its basically doing three things -

1. find the symbol given identifier name and identifier index

2. Find all identifiers from that symbol

3. Replace those identifiers with identifier with different name.

Now steps 1 and 2 is more complicated than is required for a naive example. Need step 1 so that you simulate "selecting" a specific repetition of identifier name. So for ex, we have 4 identifiers named foo. Selecting repitition index 0,1,3 renames outer foo, selecting index 2 renames inner foo. So, this also shows how rename is safe even with variable shadowing.

Now, because in step 1 we are selecting a specific repetition (say index 1), we need to start over again (as we have crossed 0) to find all the identifiers related to that symbol. This is step 2.

If we assume that we want to only select index 0,then step 1 and 2 can be merged together. Also, it takes care of not finding any identifier with that name. So, really depends on what we want to show - a basic example (which is smaller, but ignores some cases, good for reading) vs a more robust example (better for copy paste).

[bendtherules]

Yes, i also never used forEachChild before.

But its great when you want to just visit all nodes and not really return/modify anything. Also, you can cancel visiting at any point