[JS] Add generic to Table for column names

[asfimport]

Requested by @domoritz
Something like:

class Table<ColName extends string = string> {
    ...
    getColumn(name: ColName): Vector<any> {
    }
    ...
}

It would be even better if we could find a way to map the column names to the actual vector data types, but one thing at a time.

Reporter: Brian Hulette / @TheNeuralBit
Assignee: Brian Hulette / @TheNeuralBit

PRs and other links:

• GitHub Pull Request #2256

_{Note: This issue was originally created as ARROW-2767. Please see the migration documentation for further details.}

[asfimport]

Dominik Moritz / @domoritz:
One thing I realized later is that the getColumn(name) function can then guarantee to return a column rather than returning Column | null.

[asfimport]

Brian Hulette / @TheNeuralBit:
We could also do

class Table<T extends {[name: string]: any}> {
    ...
    getColumn(name: keyof T): Vector<T[name]> {
    }
    ...
}

Although I'm not sure the T[name] lookup will actually work.

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 2256
#2256