Enable incremental completion of latex symbols

If there is no whitespace between the nearest `\` and the cursor, try to complete a latex symbol or its name *instead* of a Julian name.  This allows for interactive discovery of latex names, but whitespace is required for completion of a Julia name. Note that if these completions were instead *appended* to the Julia options, they have to display without the leading \. I found that to be confusing when mixed in with the Julian names.

If the word matches a latex name exactly, it replaces it with the symbol. Otherwise, it attempts to complete the latex names. While there are some names that are prefixes to other names, I don't find this to be too jarring. It does effectively "shadow" the longer names, making them harder to discover.
Remove superfluous comparison
Merge pull request #7044 from mbauman/latex-complete

Incremental completion of latex symbols