fix: escape backslash in frontmatter regex

[Jiaoyc224]

Summary

Fix a frontmatter parsing bug in lib/index.js.

Problem

In parseFrontmatter(), the regex is built from a string:

const line = fm.match(new RegExp('^' + key + ':\s*(.+)$', 'm'));

In JavaScript string literals, \s is not an escape sequence and becomes s.
The resulting regex is /^key:s*(.+)$/m instead of /^key:\s*(.+)$/m.

This can drop the leading s from a frontmatter value when the value starts
with s and has no space after the colon, e.g. description:search GitHub repos.

Fix

- const line = fm.match(new RegExp('^' + key + ':\s*(.+)$', 'm'));
+ const line = fm.match(new RegExp('^' + key + ':\\s*(.+)$', 'm'));

Test

const re = new RegExp('^description:\\s*(.+)$', 'm');
console.log(re); // /^description:\s*(.+)$/m
console.log('description:search repos'.match(re)?.[1]); // "search repos"