Consider converting to an `autoload`-able function

[danielshahaf]

Consider converting zsh-syntax-highlighting from a sourced script to an autoloaded function. Benefits include better isolation from calling environment and zcompile byte-compliation support.

[danielshahaf]

Using autoload -U would prevent aliases in the calling environment from affecting z-sy-h. (See for example the following downstream bug report: https://bugs.debian.org/842994.)

[danielshahaf]

Note also the workers/40327 changes to autoload with an absolute path argument.

[danielshahaf]

workers/40458 mentions a useful "autoload a number of related functions" that's now possible in 5.4-to-be.

[phy1729]

Will the highlighter interface change to allow for autoloaded highlighters?

[danielshahaf]

From a mile high view, I assume that'd be an improvement.

[danielshahaf]

Once that's done we can also emulate -L and setopt warncreateglobal ${(k)zsyh_user_options[(I)warnnestedglobal} as in #413. (That construct expands to "warnnestedglobal" on zsh's that have that option and to nothing otherwise.)

[danielshahaf]

Cross-referencing #688.