build: add 'prefer-const' tslint rule

[rafaelss95]

No description provided.

[rafaelss95]

Note: This PR is here to ask if you're interested in adding this rule. If so, I can go ahead and fix all the incoming lint issues.

CC @jelbourn.

[jelbourn]

While I agree we should prefer const, I'm concerned about the huge number of line changes this would cause. It would end up causing a lot of merge conflicts without providing any major value

[rafaelss95]

@jelbourn Indeed. Maybe try to fix component by component (step by step) or just close this PR?

[jelbourn]

I'd be okay with just closing since I don't think the preference for const over let makes a big enough difference to bother

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