[FEATURE] Add thesaurus mode autocomplete for normal text files

[sujairamprasathc]

Is your feature request related to a problem? Please describe.
I'm always frustrated when using default autocomplete while editing normal text files as it only shows words already in a file.

Describe the solution you'd like
Default the autocomplete to thesaurus mode for normal files and use other modes when editing source files according to #1.

Describe alternatives you've considered
Default the autocomplete to thesaurus mode for all files, temporarily before implementing full-blown autocomplete support.

Additional context
None