Upgrade chroma to v2.3.0

[silverwind]

The behaviour of PreventSurroundingPre has changed in alecthomas/chroma#618 so that apparently it now causes line wrapper tags to be no longer emitted, but we need some form of indication to split the HTML into lines, so I did what yuin/goldmark-highlighting#33 did and added the nopWrapper.

Maybe there are more elegant solutions but for some reason, just splitting the HTML string on \n did not work.

[silverwind]

It might be possible to use chroma.Iterator directly in highlight.go so we can skip the joining and splitting of the HTML altogether and nopWrapper wouldn't be needed as well, checking...

[silverwind]

Now it uses chroma's SplitTokensIntoLines instead of homebrew line splitting and we don't need the nopWrapper any more.

It seems to work fine, but I'm unsure whether that range loop is as efficient as it can get because if flushs the writer and resets the buffer every iteration. Would appreciate feedback on whether this can be optimized.

[wxiaoguang]

Some small changes:

• remove htmlWriter (no need to have it or do flush), the htmlBuf.Reset will reuse the underlying allocated memory, so no performance problem
• rename tokensArray to tokensLines to match the function chroma.SplitTokensIntoLines
• pre-allocate memory for lines with make

and LGTM

[lunny]

make L-G-T-M work