This replaces the regex-based approach to templates with a simple non-failing and forgiving parser.

Now there are 3 types of variables:

• Regular variable, it just expands to its value, e.g. ${FILES} (BRANCH, FILES_COUNT, FILES are supported)
• Sliced variable, it expands to special sliced value limited by N, e.g. ${FILES:N} (currently only FILES is supported)
• Regex variable, same as regular variable, but before expanding it replaces regex with replacement, e.g. ${BRANCH/regex/replacement}. Supports all variables that regular variables support. No newlines allowed inside regex and replacement because that's error prone. Capture groups ($1, $2, ...) are supported, as expanding directly uses Regex.Replace(String, String).

To escape variables within text, use \ before $, e.g. \${FILES}. It is also used to escape / within regex and } within replacement for regex variables. \ for escaping is only valid in those contexts, otherwise it behaves like a regular character, e.g. C:\a\b => C:\a\b, \\ => \.

I also changed the spelling of variables to be more recognizable and "standard", but legacy spellings like files, branch_name were kept for compatibility.

Adding a variable is trivial, add an entry to s_variables for regular ones and/or to s_slicedVariables for sliced ones.
Unknown variables are silently expanded to an empty string, but I propose to add some warning logic for this.

Since there is no testing framework used for SourceGit, I didn't add a test for the template engine, but I tested it externally and it works fine. I also did some manual testing inside SourceGit, but final check is needed.

Close #704