Add `-e`/`--regex` opt-in mode — triggers v2.0.0 release

[dolph]

Goal

Add a -e / --regex flag that treats the pattern as a regex (RE2 syntax) instead of a literal string. Capture groups in the replacement work via the standard $1, ${name} syntax.

find-replace -e '(\w+)_test\.go' '$1.test.go'
find-replace -e 'github\.com/old/(\w+)' 'github.com/new/$1'

Default stays literal

Literal mode remains the default. Switching the default to regex would silently break any existing invocation whose pattern contains ., +, *, ?, (, [, \, etc. — that's most real-world patterns (filenames, version strings, URLs). Opt-in keeps every existing script working.

Why this triggers a major release (v2.0.0)

Even though the default doesn't change, this is the natural breakpoint for the regex feature set:

• New flag surface (-e/--regex) and new replacement syntax ($1, ${name}, $$)
• Combined with the prerequisites below, this is a substantial expansion of what the tool does
• v2.0.0 gives a clean documentation cut and a clear "before/after" for users reading the README

The major bump is more about communicating the scope of new capabilities than about breaking changes.

Depends on (must land first)

• Use two-phase rename to safely handle collisions and cycles #35 — Two-phase rename safety. Regex renames make collisions and cycles routine (s/foo/bar/ over a tree with both names). The safety substrate must be in place before regex mode goes live.
• Add --rename-only and --content-only scope flags #36 — --rename-only / --content-only scope flags. Regex patterns often want to target one half (e.g. rename files by pattern without scanning every file's contents). Scope flags need to exist before users start writing regex invocations.
• Build regex engine with capture-group replacement support #37 — Regex engine with capture-group replacement. This issue is the user-facing flag; the engine work in Build regex engine with capture-group replacement support #37 is what it exposes.

Acceptance

• -e / --regex flag added; literal stays default
• Replacement supports $1, ${name}, $$ (literal $), $0 (whole match)
• Invalid regex exits non-zero with a clear compile error pointing at the column
• Capture groups work identically in rename and content paths
• README rewritten for v2.0.0: regex examples, scope flags, two-phase rename note
• CHANGELOG entry describing the major bump
• Version bumped to 2.0.0
• Release tagged after Use two-phase rename to safely handle collisions and cycles #35, Add --rename-only and --content-only scope flags #36, Build regex engine with capture-group replacement support #37 are merged