Add helper for Markdown inline code escaping

[fmeum]

The helper adds escaping to a string so that it can be embedded in a Markdown inline code segment delimited by backticks.

[tetromino]

I agree with the PR in principle, but I would ask for some changes.

1. Terminology nit: GH-flavored-markdown's official name for this is "code span", so let's rename the method to markdownCodeSpan
2. Please include the outermost backticks in the function's output. The only reason to not include them would be to allow concatenating multiple strings within a single span, e.g. to have`${util.markdownCodeSpan(foo)}${util.markdownCodeSpan(bar)}` in a template - but that is impossible to do safely (consider what happens when ${util.markdownCodeSpan(foo)} outputs a string with a terminal sequence of `-s). Omitting the outermost onion layer of backticks is therefore a footgun for careless template writers.
3. We'd want a unit test in src/test/java/com/google/devtools/build/skydoc/SkydocTest.java on second thought, let's create a src/test/java/com/google/devtools/build/skydoc/MarkdownUtilTest.java

[tetromino]

Note that I'm in the process of moving the renderer code from Bazel into the Stardoc repo; if you don't have time to make those changes, I can do them myself before the move (to avoid merge complications).

[fmeum]

Note that I'm in the process of moving the renderer code from Bazel into the Stardoc repo; if you don't have time to make those changes, I can do them myself before the move (to avoid merge complications).

I made the changes you requested above, but I'm not sure what I would/would not have to do to support the move.

[tetromino]

Thanks. Looks good. I took the liberty of adding the missing srcs filegroup and fixing some stylistic nits / linter suggestions.

[fmeum]

@tetromino Thanks! Looks like there is a visibility issue with that though.