Add build to avoid using regexpu-core in runtime

[barak007]

This PR moves regexpu-core to be a dev dependency to save 380k when creating a web bundle by transpiling the usages in build time.

Also might have a minor speed up at initialization time.

Fixes #22

[codecov-commenter]

Codecov Report

Merging #23 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #23   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         6    +1     
  Lines          187       188    +1     
=========================================
+ Hits           187       188    +1     

Impacted Files	Coverage Δ
lib/parse.js	100.00% <100.00%> (ø)
lib/stringify.js	100.00% <100.00%> (ø)
lib/uni-regexp.js	100.00% <100.00%> (ø)

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[AviVahl]

heya @evilebottnawi, any chance you could check this optimization out?

It makes css-selector-tokenizer much smaller when bundled for browser usage, and with less transitive runtime dependencies.

[tomrav]

@geelen we'd really appreciate it if someone can review and merge or comment on this PR.

[barak007]

@sokra @evilebottnawi I would appreciate if someone could take a look on this PR. We are currently using a work around to avoid the huge size increase.

[alexander-akait]

@barak007 Where we use this module? Wi migrate on postcss and postcss-selector-parser

[AviVahl]

@evilebottnawi we are using it in several places in https://github.com/wix/stylable

This PR solves a size regression that affects anyone who bundles css-selector-tokenizer and runs it inside the browser. It probably also improves the evaluation time of the package.

[alexander-akait]

Looks good, can we merge and do release? It is just a patch, nothing changes, yes?

[AviVahl]

Yes, it can be a patch release. 👍

[alexander-akait]

Done, 0.7.3 is released