formatter aggressively reformats externs into a mess

background: https://forum.rescript-lang.org/t/my-main-gripes-with-rescript-constructive-criticism/6981/5

right now, the formatter is ruthless in formatting. this is usually appreciated, but for externs, it becomes messy sometimes.

imagine a file such as

@module("moduleX") external f1: someType1 => someType2 = "someFunc"

@module("moduleX") external f2: anotherType1 => thirdType = "someOtherFunc"

@module("moduleX") external f3: interestingType1 => interestingType2 = "someFuncWithALongerName"

@module("moduleX") external f4: t1 => t2 = "anotherFunc"

the formatter reformats it to:

@module("moduleX") external f1: someType1 => someType2 = "someFunc"

@module("moduleX") external f2: anotherType1 => thirdType = "someOtherFunc"

@module("moduleX")
external f3: interestingType1 => interestingType2 = "someFuncWithALongerName"

@module("moduleX") external f4: t1 => t2 = "anotherFunc"

because of the different lengths. this makes the externs really difficult to parse easily by looking at them because there are often repetitions and variants in the functions, so keeping them all on the same format is easier despite some lines being a little longer than what the formatter prefers.

let's discuss potential solutions for this.

Start with the formatter behavior and extern examples in this issue, then read the linked forum discussion for the proposed solutions and trade-offs. Done is not yet defined: the issue needs an agreed formatting rule before a newcomer can identify the implementation entry point or validation criteria.