fix: remove 'use strict'; from systemjs when strict=false

[askbeka]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:
#3100

Description

Removes 'use strict'; from Systemjs modules whenoutput.strict = false

[codecov]

Codecov Report

Merging #3101 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3101      +/-   ##
==========================================
+ Coverage   89.22%   89.27%   +0.05%     
==========================================
  Files         165      165              
  Lines        5725     5726       +1     
  Branches     1738     1739       +1     
==========================================
+ Hits         5108     5112       +4     
  Misses        380      380              
+ Partials      237      234       -3

Impacted Files	Coverage Δ
src/finalisers/system.ts	96.47% <100%> (+0.04%)	⬆️
src/finalisers/umd.ts	94.91% <0%> (+1.69%)	⬆️
src/finalisers/cjs.ts	97.43% <0%> (+2.56%)	⬆️
src/finalisers/amd.ts	97.14% <0%> (+2.85%)	⬆️

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[lukastaegert]

Very nice, also the test case and the positive coverage diff 😉!