Add execline lexer (#1393)

* Implement lexer for execline.

This commit introduces a lexer for Laurent Bercot's execline scripting language
(https://skarnet.org/software/execline) based on Pygments' existing bash lexer,
with some minor adaptations for execline's variable naming rules.

* Add versionadded note and website link to execline lexer.

* Add execline to languages.rst and example execline script

* Explicitly mark non-special characters in execline lexer as Text

* Correct execline lexer version addded

Co-authored-by: Molly Miller <sysvinit@users.noreply.github.com>

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