feat: Add 'concat' method as immutable alternative to 'append'

[cassiozen]

Useful for reusing a base query with multiple different appended statements:

Example:

const baseQuery = SQL`SELECT * FROM books`;

app.get("/", (req, res) => {
  const query = baseQuery.concat(SQL` WHERE id = ${req.params.id}`);
  // Some response
});

References #71

[codecov]

Codecov Report

Merging #72 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #72   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          35     47   +12     
  Branches        6      9    +3     
=====================================
+ Hits           35     47   +12

Impacted Files	Coverage Δ
index.js	100% <100%> (ø)	⬆️

[skyjur]

May I suggest alternative solution #79