update runmode-standalone to match core and runmode.node.js

[BrianHung]

This PR does the following:

• updates splitLines to match runmode.node.js
  This just follows changes made to runmode.node.js made in 0bc820b

• updates getMode to match core and runmode.node.js
  This just follows changes made to runmode.node.js made in 0bc820b.
  Function was mainly taken from modes.js.

• updates resolveMode to match core
  Function was mainly taken from modes.js.

• includes extendMode to match core and runmode.node.js
  This just follows the changes made to runmode.node.js made in 0bc820b
  Function was mainly taken from modes.js.

• includes innerMode to match core
  This just follows the changes made to runmode.node.js made in deaa842
  Function was mainly taken from modes.js. This is needed for markdown to work in standalone.

• includes copyState to match core
  Function was mainly taken from modes.js. This is needed for markdown to work in standalone.

• updates StringStream to match core
  This just follows the changes made to runmode.node.js made in 0bc820b, later changes made to util/StringStream, and the previous commit to runmode-standalone.

[marijnh]

Would you be interested in researching if it would be hard to run rollup on a script importing this functionality directly from src/ (maybe rearranging some of the code to make the tree-shaking work and avoid pulling in irrelevant code) to create this file, instead of duplicating the code? The current approach comes from a time before the library used any bundler or module system, and at this point it's both a constant source of problems and a bit of an embarrassment.

[BrianHung]

Sure, I can't guarantee anything though. Working through this PR, I found that the only files from src needed are src/modes.js, src/utils/StringStream.js and src/utils/misc.js so most of the core in src isn't needed.

Edit: Some progress... Right now, it's bundling everything it needs from src/ instead of importing directly.

[marijnh]

Replaced by #6314