chore: move legacy context stuff into its own object

[Rich-Harris]

This is a minor tweak that addresses part of #11297 — there's no point creating e.g. the l1 array and the l2 source in runes mode. Smaller objects are faster, and this way it's clearer what we can delete in future.

I also got rid of the redundant line comments — that's what the JSDoc annotations are for

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