Adds manually written index.d.ts typescript type declaration

[telamonian]

I've tested this PR out by yarn linking-ing it into an external typescript project. The typings seems to work well.

• 4069dec - this one adds the actual index.d.ts
• 9bd534d - minor fixups to jsdocs to match 4069dec
• 1406f41 - fixes RegularTableElement.scrollTo shadows base HTMLElement.scrollTo method #54: renamed scrollTo -> scrollToCell, so that the method can be included in index.d.ts without the typescript compiler complaining about shadowing of the base class method

@texodus 9bd534d added docstrings for the RegularTableElement private methods:

• on the genreral principle that the more (dev) docs, the better
• I added @internal @private tags to the private docstrings. In theory, that means these private docstrings will get ignored both by jsdoc and at least one declaration generator that we can experiment with later
• if you think these aren't appropriate, I can also just get rid of them

[texodus]

Thanks for the PR! Looks good!