Style guide and linter for M2 code

[mahrud]

(I thought there was an issue about this already, but I can't find it.)

We need a basic linter for M2, but there needs to be a style guide first. We can use this issue to collect suggestions. I'll start with a few. Feedback for changing these are welcome!

---

0. Break any of the rules below when necessary for readability.
1. Use 4 space indents inside code after opened (, {, [, <|.
2. Use 4 space indents inside code after for, do, list, while, if, then, else, e.g.

   
   if cond then (
   multi-line code 1
   )
   else (
   multi-line code 2
   )

if cond then result1 else result2

if cond then result1 else result2

while cond do stuff

while cond do stuff

while cond do ( multi-line stuff )

for i in range do stuff

for i in range do stuff

for i in range do ( multi-line stuff )

3. Use 4 spaces after `->` and `=>`, e.g.
```m2
longMethodName Module := Module => opts -> (n, R, M) ->
    if n == 0 then R^1 else
    if n  > 0 then blah blah M else
    error "expected non-negative integer"

longMethodName Ideal := Module => opts -> (n, R, I) ->
    longMethodName(n, R, module I)

longMethodName Ideal := Module => opts ->
    (n, R, I) -> longMethodName(n, R, module I)

H = new HashTable from {
    "very very ......................................... very long key" =>
        12,
    }

4. When plausible, use spaces to tabulate keys and values in hash tables, e.g.

   blah = method(
   Options => {
       Strategy    => null,
       LengthLimit => infinity,
       }
   )

5. Use 2 space indents inside documentation after things like Node, Key, ..., but one more or less for cases like this is acceptable:

   doc ///
   Node
   Key
   func
   (func, Module)
   (func, Sequence)
   [func, Strategy]
   Usage
   func M
   func(M,N)
   Inputs
    M:Module -- graded
   "(M,N)":
      a pair of modules
   SeeAlso
   func2
   (func, Ideal)
   ///

6. Use tabs for each 8 spaces and remove extra spaces before the tab character.
7. Insert a space after each comma in sequence of different types, e.g. (n, R, M).
8. Dropping the space is fine in sequences of same type, e.g. [x,y,z].
9. Insert a comma after the last item in hash tables, export lists, etc., e.g.

   export {
   "blah",
   "foo",
   }

10. Don't use parenthesis around a single object:

    
    blah(ZZ) := (n) -> (2*n) -- bad
    blah ZZ := n -> 2*n      -- good

dim(module(I) -- bad dim module I -- good

coker(gens(I M)) -- good coker gens(I M) -- good

-- exception: functions which accept only a single object func = (x) -> stuff

11. Put a space before and after binary operators like `= := -> => < ? | xor`

[d-torrance]

12. No trailing whitespace

[mahrud]

13. Use 2 space indents inside TEST.