type-literal-delimiter: Prefer ';' instead of ',', and lint for trailing delimiter

[andy-hanson]

PR checklist

• Addresses an existing issue: Make type literal delimiter semicolon by default #2778
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

Switches the preference to ; instead of ,.
Checks for a trailing delimiter in multiline type literals, and for no trailing delimiter in single-line type literals.

Blocked waiting on #2789.

CHANGELOG.md entry:

[rule-change] type-literal-delimiter: Prefer ; instead of ,, and lint for trailing delimiter

[nchen63]

Wait, semicolons seem unexpected here:

function f(arg: { a: number; b: string })

Yet, I get @adidahiya's comment on

type t = {
  a: number,
  b: string
}

also being inconsistent with interface. Is it worth it to make a more complicated rule and be inconsistent? I think I'd rather see all commas instead of the 1st example.

[adidahiya]

yeah I noticed that weirdness too @nchen63... I think we should add more configuration to the rule before enabling it in the built-in configs (I noticed you removed it from tslint:latest in the v5.3 release PR 👍)

how about we make single-line type literal checking opt-in for this rule? there could be a rule option called "check-single-line" that enforces the specified delimiter type?

[nchen63]

I think I'm going to make a fork that doesn't use this rule so we can think about it some more and publish based on the fork