<for> switch to tag params

[DylanPiercey]

Description

This pr updates the <for> tag to use tag params instead of the existing special syntax.

It also normalizes the for loop, and removes some unneeded functionality. Previously the <for> use a javascript like syntax that deviated in many ways.

Here are some of the previous ways you could write a for loop:
for(item in array)
for(item in callbackGeneratorFunction)
for(key, value in object)
for(i from 0 to 10 step 2)
for(item in array | status-var=loop)
for(item in array | separator=" ,")
for(item in array | iterator=someFunction)
for(item in array | array)

Migrators have been added in this PR for all of the above variants where possible. Where a migration was not possible there now may be some additional runtime warnings, but all existing syntax should continue to work until a future version of Marko. You can automatically migrate the old syntax using marko migrate.

The new <for> loop

The <for> loop in this PR does not introduce any additional syntax and leverages the new tag parameters syntax.

There are three variants, here are some examples of using the <for> loop:

with an of attribute (iterates over an array/iterable).

<for|item, index, all| of=array>
  $ const isLast = index === all.length - 1;
  ${index}: ${item}

  <if(!isLast)><br/></if>
</for>

with an in attribute (iterates over an objects properties).

<for|key, value| in=object>
  ${key}: ${value}
</for>

with a from and to attribute (iterates between a range of numbers).

<for|i| from=0 to=10 step=2>
  ${i}
</for>

Checklist:

• I have read the CONTRIBUTING document and have signed (or will sign) the CLA.
• I have updated/added documentation affected by my changes.
• I have added tests to cover my changes.

[coveralls]

Coverage decreased (-0.1%) to 90.462% when pulling 565dc73 on for-tag-params into de397d4 on tag-param-core-tags.

[codecov]

Codecov Report

Merging #1238 into tag-param-core-tags will increase coverage by 0.03%.
The diff coverage is 89.35%.

@@                   Coverage Diff                   @@
##           tag-param-core-tags    #1238      +/-   ##
=======================================================
+ Coverage                90.56%   90.59%   +0.03%     
=======================================================
  Files                      337      334       -3     
  Lines                    12356    12339      -17     
=======================================================
- Hits                     11190    11179      -11     
+ Misses                    1166     1160       -6

Impacted Files	Coverage Δ
src/taglibs/migrate/util/parseFor.js	94.33% <ø> (ø)
src/compiler/util/removeComments.js	100% <ø> (ø)	⬆️
src/compiler/CompileContext.js	92.28% <ø> (ø)	⬆️
src/compiler/Builder.js	92.05% <100%> (+1.39%)	⬆️
src/runtime/helper-forEachProperty.js	93.75% <100%> (+1.44%)	⬆️
src/compiler/Normalizer.js	98.33% <100%> (ø)	⬆️
src/runtime/helpers.js	94.06% <66.66%> (-4.05%)	⬇️
src/compiler/ast/ForEach.js	78.57% <66.66%> (-9.9%)	⬇️
src/taglibs/core/for-tag.js	73.91% <72.72%> (-18.4%)	⬇️
src/compiler/ast/ForEachProp.js	82.35% <75%> (-6.84%)	⬇️
... and 41 more

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[mlrawlings]

?

[mlrawlings]

Looks like a few cases aren't handling the status var correctly

[mlrawlings]

${loop.getIndex() + 1}) of ${loop.getLength()}

[mlrawlings]

loop.isLast()