Validate compiled and compressed JS using grunt

[jorbin]

In the past we've occasionally run into issues with uglify or other minifyers. ​Grunt JS-Validate parses JS and thus alerts us to these occasional errors. This does add ~1 second to grunt build, but that's not so bad considering we are validating 276 javascript files.

[wonderboymusic]

In 26977:

Add jsvalidate as a dependency to the build. To get the new module, run npm install in the root of the project.

Fixes #26615.
Props jorbin.

[TobiasBg]

The src checks too many files here. BUILD_DIR + '/**/*.js' includes JS files of plugins (or other stuff in wp-content).
Not only does this cause extra work during a grunt build, but it can also raise errors where .js files are used for something else (e.g. not holding JS code, but JSON data).

[TobiasBg]

Besides what I mentioned above, this also tries to validate JS files in npm modules of plugins in wp-content (e.g. if they also use grunt) -- which results in errors like

>> Line 2: Unexpected token :
Validating build/wp-content/plugins/<my_plugin_slug>/node_modules/grunt-contrib-csslint/node_modules/csslint/cli.js  

Thus, we should exclude wp-content here (for which I uploaded a patch). (Drawback: Core themes would aso be exlcuded.)
Other options could be to just jsvalidate .min.js files (which is what this ticket was intended for, I think), although that would also check .min.js files in plugins.
An even better solution might be to check just the files that were created by uglify. My tries with <%= uglify.core.dest %> failed however.

[TobiasBg]

Don't jsvalidate files in wp-content

[TobiasBg]

Just saw that this should go to the 3.9 Milestone, too. @SergeyBiryukov ? :-)

[wonderboymusic]

In 27054:

Ignore .js files in wp-content when running jsvalidate during grunt build. See [26977].

Props TobiasBg.
Fixes #26615.

[kadamwhite]

In 56247:

Build Tools: Remove outmoded jsvalidate task from Gruntfile.

grunt-jsvalidate was added to catch situations where minification introduced parsing errors in Internet Explorer.
IE is no longer supported by WordPress, and the grunt task itself has not been maintained for a decade.
Removing this task simplifies the build and avoids false-positive warnings where the outdated tool flags modern JS syntax incorrectly.

Props westonruter, joemcgill, jorbin, youknowriad, desrosj, swissspidy.
Fixes #58645. See #26615.